As per #1731, this PR adds a dependencies version policy to the docs. It also includes a file containing a list of all configure-time deps.

Obsoletes wiki page.

Add a first round of developer documentation, evolving from the older `internal` part of the docs.

- Add a callback to be executed between epochs.
- Add example: print progress indicator

Half-half dev and user docs on NMODL optimisation. Actually apply that advice in hh.mod

* Some badges corrected (build) and added (Gitter)
* Minor Spack backports
* Small updates to `release.rst`

Closes #1822. Adds a separate tutorial for extracellular signal predictions using LFPykit to the docs. Adds some minor improvements to the corresponding python example file. Introduces Intersphinx for linking to other documentation.

* Update VERSION to `v0.6.1-dev`
* Updates to `doc/release.rst`
* Update Zenodo citation info for v0.6
* Update copyright info and maintainers in `spack/package.py` from upstream
* Fix error in build-badge docs

* 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.

# Changes

- `cable_cell_global_properties` now holds a catalogue directy
- fixes for tests, examples, and python bindings accordingly

# Linked Issues
Fixes #1446 

- 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/auditwheel#363
    - Python Wheels are tested as part of the Github Action 
    - Add nml and bundled status to config().

* Pyarb specific.
* `proc_allocation_shim()` throws error if user sets threads to zero.
* `arbor.context` constructor accepts `threads` set to `"avail_threads"`, which sets the number of threads to `arbenv::thread_concurrency()`
  * This introduces a dependency on arbenv for Pyarb.
* Docs and tests added.

Fixes #1692 

- Add support for profiling externally loaded mechanisms.
- Move the profiler calls out of the generated C++ code into the `arb::mechanism` methods. 
- Remove the `--profile` flag from the modcc flags. 
- Replace `_` delimiter with `:` in profiler. 
- Replace `const char*` with `const std::string&` for profiler region representation.
- Profiler may now contain empty regions that were registered but not profiled, so add some code to filer those out when generating final profile.

- 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 

- Implement a new constructor for `domain_decomposition` from a vector of `group_descriptions`. The new constructor performs various checks on the given `group_descriptions` and throws an appropriate exception if any problem is found. It is the only available constructor for `domain_decomposition`.
- Make the various members of `domain_decomposition` private and implement their getters.  
- Expose the `domain_decomposition` constructor to the python interface through a `partition_by_group` function.
- Add unit tests. 
- Add documentation.

Documents the release cycle.

Refactor the CV-cable logic from `fvm_layout.cpp`. Specifically, move `cv_geometry_from_ends` into the new public header `cv_data.hpp` and rename it to `cv_data_from_locset`, and move `pw_over_cable` into another new public header `util/piecewise_over_cable.hpp`. 
Create a new public class `cell_cv_data`, similar to the private `cv_geometry` but which only carries the cv information for a single cable-cell. Add a new constructor to `cv_geometry` which takes `cell_cv_data` as an argument. 
Add new public function `intersect_region` that takes as arguments a cable-cell, a region on that cell and a `cell_cv_data` object and returns a vector of CV indices and proportions belonging to the region. 
Add docs, python wrapper and unit tests. 

Fixes #1720. 

* Add documentation for collaboration on a PR using just git
* Add the stage of Issue creation to PR workflow.
* Correct header level in test docs

Fixes #1753

Extend the `pwlin_place` interface to find the location of a morphology that is closest to a 3d coordinate.
* extend `arb::pwlin_place` interface
* python wrapper
* unit tests
* documentation

Addresses a feature request by @Helveg that I don't think ever had a ticket assigned to it.

Fixes #1661 and #1108.

* Consolidate default_concurrency, default_gpu functionality into arborenv/default_env.hpp, .cpp.
* Add new arborenv function `default_allocation()` that uses environment variables ARBENV_NUM_THREADS and ARBENV_GPU_ID to create an `arb::proc_allocation`, with a fallback to `thread_concurrency()`.
* Use arborenv-specific exceptions instead of generic std::runtime_error etc.
* Update examples, docs to suit.

Fixes #988 (except for reworking all the unit tests!)