Update `set_ion` method of `cable_global_properties` to allow the addition of new ions

* Remove redundant `initial_ion_data` specification.

* Use CMake check_cxx_source_compiles and check_cxx_source_runs to detect how std::filesystem
is enabled. The probes are executed first, if none succeeds, we use link-only to cover the possibility that we
are cross-compiling.

Preliminary work towards the mechanism ABI.

Unification of the `public` and `protected` interfaces of the mechanism classes.
These interfaces will be merged into the public API ('public' as in: exposed via `arbor/include/arbor`). 

Interface methods `nrn_` have been renamed
- `nrn_init`: `init` (would have preferred initialize, but that is taken)
- `deliver_events(event_stream)`: `apply_events` (to avoid a name clash)
- `nrn_state`: `advance_state`
- `nrn_current`: `compute_currents`

Additional changes:
  - Remove typedefs for index_type etc
  - Shuffle `ion_state_view` to public interface
  - Migrate `copy_extend` to a proper place out of `multicore/mechanism.hpp`. Maybe `memory` or `range_util`.
  - Remove `vec_t_to_`

* Update `README.md`: moved citation to `doc/index.rst`.
* Advertise Py3.9 compat PyPI in setup.py
* Fix typos.

* 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.

`/doc/scripts/divio_docs_theme` has the contents of the tarball from PyPI, slightly modified (custom fonts removed, font-awesome reduced to woff2, logo enabled, divio banner/analytics removed.).

Remove transmogrify layer from s-expression parser.

The transmogrify layer isn't needed if asc files won't be parsed as s-expressions.
It complicates and slows down parsing of s-expressions.

- Allow SOLVE statement anywhere in the BREAKPOINT block.
- Add unit test.

Fixes #1384.

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

* add support for windows new line in s-expression parser
* make small optimization that reduces time to read large strings by 20%
* fix another small bug with handling error states.

Fixes #1381

* Remove check for strict linearity in current contribution in modcc, as it is no longer necessary.

* Refactor the functions from `algorithms.hpp`:
  - `mean`: moved to `profile/meter_manager.cpp`
  - `make_index`: moved to `util/index.hpp`
  - `is_strictly_monotonic_increasing`: moved to `/backends/gpu/forest.hpp`
  - `has_contiguous_compartments`:  moved to `/backends/gpu/forest.hpp`.
  - `branches`: moved to `/backends/gpu/forest.hpp`.
  - `expand_branches`: moved to `/backends/gpu/forest.hpp`.
  - `tree_reduce`: moved to `/backends/gpu/forest.hpp`.
  - `is_minimal_degree`: moved to `tree.hpp`.
  - `child_count`: moved to `tree.hpp`.
  - `in_interval`: not used, deleted.
  - `is_strictly_monotonic_decreasing`: not used, deleted.
  - `all_positive`:  not used, deleted.
  - `all_negative`: not used, deleted.
  - `find_branch`: not used, deleted.
  - `is_unique`: not used, deleted.
  - `binary_find`: not used, deleted.
* Remove `make_parent_index` from `tree.hpp`: not used. 
* Replace `make_index` usages with `make_partition`.
* Add unit tests to `tes...

- Make reversal potential `e` a global parameter of `pas` to avoid inconsistency 
  (reversal potential is represented by a RANGE parameter, despite it not being a conductance multiplier see #1052)
- Adjust default value of `e` to -70mV (as in Neuron)
- Streamline docs,  which use `passive`, `el`, and `gl` instead of `passive`, `e`, and `g`.

Closes #1052

- remove repeated CMAKE_BUILD_TYPE
- bump lcov to 1.15
- use new --exclude flag to skip generated sources

With gcc 8 as a compiler + a new version of lcov (#1350) codecov started tracking generated source files and coverage dropped to < 60%. Not sure if this is a bug in lcov or not. I've added these new flags for lcov: `--exclude "*/ext/*" --exclude "*/test/*" --exclude "*/mechanisms/*"` to fix it.

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.

* Sync Cable Cell page names and ordering between sections (concept, python, cpp). Except cpp cable cell page. Is still one big page.
* Move CV definition across categories to consistent location.
* Move decor to separate page (cpp currently missing decor documentation) because it's a top-level concept that should be in the TOC like the other ingredients of the cable cell.
* Slight rework of Concept intro and Cable Cell pages.
* The Cable Cell page is now basically an intro, and lay out how the other pages/concepts relate to each other.

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

* fix nmodl comment parsing
* proper handling of \r
* fix segmentation fault when api_state is null
* add unit tests

* add function unit parsing
* add unit tests

Mirror only master branch and tags.

Caused by a typo. The correct version is being used in the non-simd printer.

Reomove Travis CI integration.

* remove Travis yaml and build scripts
* remove Travis badges from README and RTD docs
* replace Travis badge with GitHub Actions badge
* add a RTD badge to the README

Fixes #1338.

* Make Gitlab CI push images to CSCS' JFrog registry reg.giuv.cscs.ch so that sarus can pull images on Daint again. 
  This URL is probably temporary until JFrog officially goes into production later this month.
  Unfortunately JFrog is currently behind the firewall.
* Upgrade g++ to g++8 on the Docker image.
* Only require libxml when compiling with NeuroML support.

* Move contents of `arbornml` to to `arborio` 
* `arborio` will be built with NeuroML2 morphology support if `ARB_CMAKE_NEUROML` is set. 
* Wrap `arborio::neuroml` and `arborio::morphology_data` for pyarb.
* Add documentation.
* add `neuroml` flag to `setup.py`

Fixes #1256
Fixes #1234

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

Fixes #1340.