- 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.

Bump version for release

* Add a GH workflow to build and run Arbor tests and examples with Clang sanitizers.
* Fix some uninitialized struct members; missing `override` qualifiers.

* Exit with correct exit code from Python test runners.
* Fix unit tests that were using older APIs.

* Install gcc-8 and clang-8 in GitHub Actions Ubuntu image, as these had been removed in an update 29.03.2021. See https://github.com/actions/virtual-environments/issues/2950

* Add Python version of brunel.cpp example.
* Enforce consistency in units: both C++ and Python APIs use unit `kHz` for all frequencies.
* Align Python version `poisson_schedule` signature with C++ equivalent.
* Update unit tests to suit API changes.
* Update documentation to suit, along with minor additions related to networks and interconnectivity.
* Add `brunel.py` to `basic.yml` tests

- Add _running_ C++ examples to standard CI runs (compiling has been in since the start).
- Fix `bench.cpp` to set return value to 1 if exceptions are caught.

* Auto-generate binary Python wheels, triggered by git tags 'v*'.
  * Wheels are generated for Python versions 3.6-3.9, for Linux using the [PyPA manylinux-2014 image](https://github.com/pypa/manylinux) and MacOS using macos-latest (10.15 at time of writing).
  * The Action generates `sdist` for any other platforms.
  * The cibuildwheel action automatically prepares the wheels using [auditwheel](https://cibuildwheel.readthedocs.io/en/stable/options/#repair-wheel-command), which means bundling of external deps (e.g. libxml2 for NeuroML).
  * The output is all that's needed for publication of a new version to (Test)PyPI.
* `setup.py` defaults `neuroml` to on.
* A small change to `CMakeLists.txt` was required to build the wheels: based on this [Pybind hint](https://pybind11.readthedocs.io/en/stable/compiling.html#findpython-mode) `Development.Module`is now required, and not `Development` when CMake searches for Python. I confirm this change is needed to make Arbor build in the manylinux docker images, but I do not understand what the change implies, other than that we now don't link to `libpython`.

* Tutorial ring network added. A few typos fixed, crosslinks fixed.
* MPI tutorial added.
* Improved SVG generation, added separate label scaling, increased default label size.
* Bugfix: didnt generate cable region in gen-labels.py used in `concepts/morphology.rst`.
* Termified Interconnectivity page.
* Termified probe nomenclature page. Will be moved in [this PR](https://github.com/arbor-sim/arbor/issues/1353) from python/simulation to concepts/probes_samplers.

* add neuroML example
* no test, because GH actions arbor is built without neuroML support

Use TCP BTL for MPI in GitHub Actions to fix sporadic SEGFAULT on MacOS.

See open-mpi/ompi#6518

Adda Linux pipeline with SIMD and not MPI to GitHub Actions.

Online STDP is implemented in the synapse and
and demonstrated by recording an STDP curve in
an example.

Mirror only master branch and tags.

Add more GitHub actions CI configurations
* min and max versions of ubuntu, clang and gcc for linux
* MacOS tests with min and max clang versions.

Fixes #1337

* Add the ability to use json and pybind11 libraries installed on the system instead of the
  vesrsions of those libraries bundled as submodules in the Arbor repository
  * Turned on by default
  * Toggled using the new `ARB_USE_BUNDLED_LIBS` CMake flag
* Update documentation for installation
* Update CI and pip workflows to opt in to the bundled libraries

Revert a change introduced in #1282 that required the python module for arbor to build the documentation.

This breaks the readthedocs build, where arbor is not available.

* Remove arbor dependence from `doc/conf.py`, reverting to the original "out of core" approach
* Fix other recently introduced issues that lead to CMake errors when configuring the documentation
  when the Python target is not configured.
  * The CMake variable `PYTHON_EXECUTABLE` wasn't set, but was required to make calls to the
     Python interpreter.
  * Instead of relying on pybind11 to find Python, explicitly detect it using `find_package(Python3 ...)`
  * Make Python3 a hard requirement when building the Python wrapper
  * And a soft requirement otherwise, that will fail only when the docs are built.

* Add 2 new examples to `python/examples` and their corresponding tutorials in the docs. 
    1. `single_cell_detailed.py` : a slightly more complex example using `single_cell_model`
    2. `single_cell_detailed_recipe.py`: equivalent to `single_cell_detailed.py` but using a recipe. 
* Change `load_swc`-> `load_swc_arbor` for consistency.
* Run Sphinx with specified PYTHON_EXECUTABLE.
* Set PYTHONPATH env var in doc/CMakeLists.txt, such that it propagates correctly, including locally.
* Build pyarb BEFORE html such that docs build can import arbor in Github Actions script.

Python tests were failing because pyarb was not built

First step of replacing Travis with GithubActions

Here, only a basic set of tests has been added

- Build arbor with MPI and Python
- Compile examples
- Run unit tests, incl distributed.
- Run python tests, incl distributed.
- Run python examples.
- Build documentation.