Fixes issue #1529.

* Adapt CSS for the HTML5 transition in docutils 0.17
* Fix some reST markup errors
* Add meta-tag to enable indexation control at Google (current indexation appears at least a year old).

Fix mistakes reported by @Helveg in the Python tutorial docs. 

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

* Use a struct with meaningful field names instead of a `std::pair` to represent elements in `pw_elements`.
* Describe explicitly the semantics of the zip operation.
* Update implementation and add unit tests to suit.
* Add comment explaining `pw_elements::equal_range`.
* Remove parameter names from unused parameters in `impl::piecewise_pairify`.

* Correct bounds check order in communicator.
* Use `.at()` in test recipe to catch failures in bound checking.

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

* Remove std::filesystem to workaround OSX features; use `std::string` in interfaces expecting a path.

Set option `-mcpu=aarch64` for target `aarch64-apple-darwin`

In response to:

https://gizmodo.com/u-s-federal-investigators-are-reportedly-looking-into-1846707144

Rationale:
* CodeCov didn't detect the breach for two months
* They took a further 2 weeks to alert users after that
* Their efforts to address the issue after that have been painful to watch
  * https://github.com/codecov/codecov-action/issues/281#issuecomment-823407167

* fix mech_vec
* fix fvm_discretize
* small fixes

* Fixes incorrect `current-clamp` generated s-expression. 
* Adds unit test.

Fixes #1486

* Move `assign` out of the `detail` namespace in the SVE SIMD implementation code.

* Add Zenodo citation snippet.
* Correct Nora's last name, which is Abi Akar.
* Small css fix for change in docutils, see: https://github.com/readthedocs/sphinx_rtd_theme/issues/1115

Fixes #1297.

* `i_clamp` constructors take new optional phase argument.
* Pulse or box envelope clamps are now provided by the static method `i_clamp::box`, taking onset, duration, and amplitude parameters.
* Bump pybind11 version so we can have keyword-only arguments in the Python API.
* Make frequency and phase keyword-only arguments to Python `iclamp`.
* Change frequency units for clamps from Hz to kHz for consistency with other interfaces.

Fixes #1454.

* update and simplify the `load_swc_neuron` reader to match the simplified and more correct interpretation in NEURON 8
* update the documentation on the interpretation, and add details about how it still differs from the neuromorpho descriptions. 
* update unit tests to reflect new interpretation
* add a unit test that reproduces a complex case from the NEURON test suite.
* remove the `load_swc_allen` C++ and Python calls
* add links to NeuroMorpho site in the documention

Fixes #1445.

Always install Python module inside CMAKE_INSTALL_PREFIX, unless the user explicitly sets the ARB_PYTHON_LIB_PATH CMake variable.

Update documentation on installing and building the Python packing using CMake to have a detailed example of how to install the Python module inside a virtual env.

Fixes #1403

* Correct the installation page.
* Custom css is not supported by theme, so a css snippet improving table formatting was moved to the theme css.
* Non cable-cell concept docs were wrong, improved formatting
* Some typos

Co-authored-by: @clinssen 

Replicate the spack `package.py` from the main spack repository into the arbor repo.

Some small updates were made:
- Correctly provide a url for the GitHub tagged release
- Specify minimum pybind11 version 2.6

Fixes #1465 

* 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

* Implements parsers and writers in `arborio` for the s-expression based Arbor Cable-Cell Format defined in #1336.
* Moves `s_expr.hpp` to the public interface.
* Adds python wrappers to support the features in pyarb.  
* Adds unit tests.
* Adds docs.

Fixes #1336
Partially addresses #1233

Co-authored-by: C.A.P. Linssen <charl@turingbirds.com>

* Fix the regular schedule generated in `single_cell_model::run`
* Adjust frequency of probes in python examples to the correct unit (KHz). 

Fixes #1457

Remove `gid` attribute of target of a connection, and local gap-junction site of a gap-junction connection (#1467)

* Change the type of the target of a `connection` from `cell_member_type` to `cell_lid_type`. The target cell's gid is always known in both the recipe and simulation.
* Change the type of the local site of a `gap_junction_connection` from `cell_member_type` to `cell_lid_type`, and make it such that the first argument of the connection is the peer site, and the second the local site. The local cell's gid is always known in both the recipe and simulation.
* Change the type of the target of an `event_generator` from `cell_member_type` to `cell_lid_type`. The target cell's gid is always known in both the recipe and simulation.
* New `cell_spike_events` and `cse_vector` for `simulation::inject_events`.
* Simplify recipe sanity checks and remove associated exception types. 
* Fix unit tests, examples, docs

* Adds a `neuroml_options` namespace with enum for use with `neuroml::morphology(...)` and `neuroml::cell_morphology(...)`.
* Add support for a 'spherical' root segment with option `neuroml_options;:allow_spherical_root`; a zero-length root segment with identical proximal and distal radius will then be converted into an area-equivalent cylinder in the resultant morphology.
* Amend `place_pwlin::all_at` so that locations on a zero-length segment with different radii at the endpoint will give an mpoint for either end.
* Add `allow_spherical_root` boolean optional parameter to Python NeuroML morphology methods.
* Update docs; C++ unit tests.

Fix orientation of ASC somas from along z axis to along the y axis.

As per the rules followed by Arbor's SWC parsers, and prescribed in:
http://neuromorpho.org/SomaFormat.html

Addresses #1445