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

* Fix semantic passes and error visitor expression coverage in modcc.

* Add a method to `event_generator` that returns all of its targets. 
* When the simulation is constructed, check the targets of each event generator, and throw an exception if an the `gid` or `lid` is invalid.
* Add corresponding unit tests.

* Make net_receive and post_event APIMethods in modcc.

* Add `nml_` prefix to NeuroML-related classes in arborio.
* Separate `arborio/xmlwrap` from NeuroML exceptions.
* Put xmlwrap functionality in its own namespace.
* Have xmlwrap property access errors returned by util::expected, so that the corresponding NeuroML exceptions are strictly the
responsibility of the NeuroML parsing code.
* Rename NeuroML implementation and support sources and headers.
* Place implementation classes for Neurolucida ASC parsing in an anonymous namespace.

Fixes #1441.

Fix bug attaching branches to the soma in ASCII file descriptions.

Branches in an ASCII description attached to the soma were incorrectly creating a segment
from the center of the soma to the first point in the branch.

Amend segment cables for zero length branches.

Ensure cables corresponding to segments cover even zero length branches, by setting the segment cables to (b, 0, 0) for all but the last segment in a zero-length branch b, and setting that last segment cable to (b, 0, 1).

Fixes #1442 

* Add on-components to label_parse.cpp, and to corresponding unit test.

Basic support for reading morphologies from Neurolucida ASCII (.asc) files.

Added to the arborio C++ library, and wrapped similarly to the other arborio features in Python.

Fixes #1429

* 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.

- Use parameter packs for multicore mechanisms.
- Adjust unit tests and modcc.
- CPU and GPU use the same mechanism_ppack_base.
- CPU code uses a split similar to GPU with `mech_XYZ_foo` and `kernel::mech_XYZ_foo.
- Make mechanism_ppack_base header public.
- Tracing macros for modcc code gen.

* Add start time to `epoch` struct, rename time fields to avoid confusion with tfinal parameter to `simulation_state::run()`. Default constructed epoch now has id -1.
* Replace `t_`, `epoch_`, `min_delay_` timekeeping in simulation_state with `epoch_` and `t_interval_`: `epoch_` holds most recently simulated interval.
* Split event_lane population out of exchange task.
* Document in code comments the sub-task dependencies in `simulation_state::run()`.
* Rewrite main simulation loop to abide by pre- and post-conditions that ensure correct behaviour from successive calls to
`simulation_state::run()`.
* Replace double buffer for `simulation::local_spikes_` with array, analogous to `event_lanes_`.
* Update cell groups, unit tests for new epoch interfaces.
* Remove `util::double_buffer` and test.
* Remove unnecessary includes in `mc_cell_group.hpp`.
* Add unit tests for simulation global spike callback and restart operations (i.e. successive calls to `simulation::run()`).
* Fix for `lif_cell_group::reset()` error leading to out of bounds access.

Fixes #873 and #1426.