* Python Wheel workflow uses `ubuntu-20.04` to fix the build
  * In #2058 I forgot to update `ciwheel.yml`
* Why this is necessarily: still have not figured it out. `cibuildwheel`
builds and tests in it's own container...
  * [x] https://github.com/brenthuisman/arbor/actions/runs/3704419712
* Small change to make `musllinux` wheels build. They don't pass testing
yet, so I don't add them to the workflow but that can be solved/added
[later](https://github.com/pypa/cibuildwheel/issues/1376)
* The reason for using `ubuntu-20.04` in the Spack workflow should [no
longer exist](https://github.com/actions/runner-images/issues/6724).

See the same jobs fail, which turn out to execute using different Python
versions:
-
https://github.com/arbor-sim/arbor/actions/runs/3618204857/jobs/6097794385
-
https://github.com/arbor-sim/arbor/actions/runs/3630819640/jobs/6141257441

Switch XML parser to pugixml; one less runtime dependency and one less blocker to WASM via emscripten.

Closes #1861 
Closes #1783 

- arbor-build-catalogue (a-b-c) does no longer need to be in a fixed location
- nor do we statically fix the configuration for a-b-c
- instead, we rely on `arbor.config` to read the relevant default values
- each value can be overriden, if desired
- added many more values to the configuration
  - gpu type and arch
  - paths and prefix 
  - CXX
- report default settings for better diagnosis
- implement a fallback for prefix if that does not exist; in particular for the amazing skbuild.

In essence you can now use a-b-c as a standalone tool, as long as you have a properly 
configured py-arbor.

Example output after removing `_skbuild`
```
Warning: prefix '/Users/hater/src/arbor/_skbuild/macosx-11.0-x86_64-3.10/cmake-install' does not exist, falling back to '/Users/hater/src/arbor/.direnv/python-3.10.6'.
usage: arbor-build-catalogue catalogue_name mod_source_dir

Generate dynamic catalogue and build it into a shared object.

positional arguments:
  name              Catalogue name.
  modpfx            Directory name where *.mod files live.

options:
  --raw raw [raw ...]
                    Advanced: Raw mechanisms as C++ files. Per <name> the files <name>.hpp, <name>_cpu.cpp (if CPU
                    is enabled) must be present in the target directory and with GPU support also <name>_gpu.cpp and
                    <name>_gpu.cu.
  -v, --verbose     Verbose.
  -q, --quiet       Less output.
  --cpu CPU         Enable CPU support.
  --debug [path]    Don't clean up the generated temp cpp code. Can be a target path for the generated code.
  --gpu gpu         Enable GPU support
  --gpu-arch gpu_arch
                    Enable GPU support; default=-
  --cxx cxx         Use this C++ compiler; default=/usr/local/opt/llvm/bin/clang++.
  --prefix prefix   Arbor's install prefix; default=/Users/hater/src/arbor/.direnv/python-3.10.6.
  --bin bin         Look here for Arbor utils like modcc; relative to prefix, default=bin.
  --lib lib         Look here for Arbor's CMake config; relative to prefix, default=lib.
  --data data       Look here for Arbor supplementals like generate_catalogue; relative to prefix, default=lib
  -h, --help        Display this help and exit.
```

- Use ninja instead of make.
- Use ccache with a per-config cache key.

Co-authored-by: Thorsten Hater <24411438+thorstenhater@users.noreply.github.com>

- GCC: goes up to 9 from 8, needed by CUDA 11
- CUDA: goes up to 11 from 10
- Clang: 10 up from 8

Closes #1815 

- bump OpenMPI version in hope of fixing the intermittent memory corruption
- bump Ubuntu and clang versions in `sanitize.yml` workflow.

- Bump CI actions to latest (cache, checkout, cmake)
- Bump mimimal compiler versions
- Bump minimal test OS to Ubuntu 20.04 LTS
- gtest as submodule (version 1.12.1)
- fixes heap corruptions
- update gtest macros, include paths

Co-authored-by: Thorsten Hater <24411438+thorstenhater@users.noreply.github.com>

* Add pybind11 version to config.
* Add version check for PB11.
* Bump CMake a tiny bit.

- It's actually a Github cache trick: https://github.com/actions/cache/issues/342#issuecomment-673371329
  - First run: 1h 7m 38s 
  - Second run: same duration. Hmm....
  - Third run: ~10 minutes!
  - 4th: ~7.5 min.
- Bump in configs: macos 10.15 will disappear in a month: https://github.com/actions/virtual-environments/issues/5583
  - Bump clang-max to 14, gcc-max to 11
  - macos-min to 11, macos-max to 12.

- Generate timestamped VERSION once, to properly group on Test.PyPI. Example:
  - https://github.com/arbor-sim/arbor/actions/runs/2696804929
  - https://test.pypi.org/project/arbor/0.7rc20220719101015/
- Update/simplify release documentation.
- Update to v0.7

* - Weekly CI Python wheel build pushes to Test.PyPI.org
    - Easy availability at https://test.pypi.org/project/arbor/
- Small change in documentation around VERSION
- Delete superfluous setup.py file.

Move Python build to `pyproject.toml`, bump Python minver to 3.7, fix macos wheel generation (#1916)

* Failing Macos Python wheel builds fixed.
* Macos Python wheels now come with dual-arch (x86-64 and arm64)
* Moved (nearly) all Python build instructions to `pyproject.toml`
  * Enables 'build isolation', and need to specify build-deps only once, no need for users or CI scripts to pre-install them.
  * Enables editable `pip` installs (`pip install -e ./arbor`)
  * Compatible with 'build frontends' `pip` and `build`.
  * Passing CMake options actually got shorter
* Drop Python 3.6 support.

Upstream bug: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1006615



Currently affects Ubuntu 22.04 users.

Co-authored-by: Simon Frasch <simon.frasch@cscs.ch>

Exports all symbols so arbor can be compiled as set of shared libraries. 

In order to export all symbols correctly, one macro per library and one global macro are added. The content of the macros is determined at configure time depending on build variant (static/shared), compiler, and platforms (linux, mac os) and goes into the library's include directory as `export.hpp` when installed (at build time it resides at cmake's temporary build directory). The per-library macro is named `ARB_LIBNAME_API` and goes in front of to-be-exported symbols. The global macro is `ARB_SYMBOL_VISIBLE`. 

This PR adds the annotation in all of the places where it is required. Most of them are in the public headers (and corresponding sources) but some are also added in internal headers, which were required for the unit tests to link properly.

Fixes #1752

* bump version to v0.6-rc
* Update `MANIFEST.in` and `scripts/run_python_examples.sh`
* Update Release instructions `docs/contrib/release.rst`
* Fix missing quotes in `python/example/single_cell_nml.py`
* Add testing to sdist creation in `ciwheel.yml`
* Rename `basic.yml` to `test-everything.yml`
   * Remove Python examples step from matrix (mostly because plotting takes much CPU on macos)
   * change to build /w nml
   * add test of pip builds (`pip install` in addition to CMake builds)
      * Python examples are tested here, once is enough.

- build-catalogue
  - now installed by pip
  - enable GPU-backends
  - isolate from being installed in a non-default dir
  - rename to arbor-build-catalogue
  - allow hand-written C++ mechanisms to be linked in the same catalogue.
- CMake
  - enable PIC on arbor always to allow linking to shared objects
  - use relative paths when configuring a-b-c, to make relocation less of a problem
- Python
    - ensure a-b-c is installed by pip, along with headers and libarbor.a
    - throw errors if tools (cmake scripts, arbor package, modcc) missing
    - transition setup.py setuptools -> skbuild.
- Wheels
    - Add NML2 support to wheels
    - scripts/build-wheels.sh builds wheels, in principle valid for submission to PyPI, on your own hardware. This should be kept in sync with .github/workflows/ciwheel.yml
    - scripts/patchwheel.py corrects the rpath in the libraries in the wheels, working around a bad interplay between auditwheel and skbuild, see pypa/auditwh...

- Bump CMake to 3.18 for better CUDA support.
- Clean-up our use of said support.
- Remove old cruft and workarounds.
- Maintenance in CI: enable MacOS 11; bump CMake versions to 3.18 / 3.22; bump bors CMake to 3.18

Fixes #1790 

* Create a script that generates `diff.log` if git submodules have tags more recent that the commit they're currently at.
* A Github Action runs this script monthly.
* _If_ updates found, Github Action creates an Issue like so: https://github.com/brenthuisman/arbor/issues/9
* Actually updating the git submodule and Spack file are still manual

First step in addressing #1731

* add a test that tests that a catalogue built separately can be loaded via the Python interface
* further simplification of dynamic library support
  * move all platform-specific code into the cpp implementation and out of header.

- `make install` in CI
- make `build-catalogue` play nice with Python 3.6
- execute `build-catalogue`

* Run examples via scripts (closes #1566)

- make spack ci run use arbor version information
- Cache Spack for GH Actions

Add clang sanitizer passes to GH CI.

Run only on Linux and Spack/develop

closes #1534

* Remove test_simulator.py.
* Add missing python unit test file and fix errors.
* Run python unit tests in verbose mode to display which were skipped.

Don't pass any architecture by default in setup.py to allow for building binary wheels for targets like x86_64.

Build Python package with 'none' arch by default.
Bump version to 0.5.2.