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.

Wrap place_pwlin interface to allow querying rich geometry information from cell morphologies.

* Add Python bindings for the `place_pwlin` and `isometry` classes.
* Add equality test for Python `mpoint`.
* Add unit tests for new Python interfaces.
* Split C++ API morphology documentation into its own file.
* Add C++ API and Python API documentation for `place_pwlin` and `isometry`.

Extend Ptyhon recipe wrapper to support setting of global cell properties.

* Rename `pyrecipe::get_probes` to `py_recipe::probes`
* Remove `global_properties_shim` : It's easy to make mistakes with the class, as it holds a `mechanism_catalogue` and a `cable_cell_global_properties` which holds a pointer to the catalogue. This would likely have caused issues with the users.
* Expose `py_recipe::global_properties`

Make the `cable_cell` interface read only by passing the decorations
to its constructor in a new `decor` type.

C++ library
* Remove the paint/place/set_default interface from `cable_cell`
* Create a `decor` type that:
  * stores lists of paintings and placings
  * stores a set of cable cell parameters
  * uses the `paint`, `place`, `set_default` interface that was deprecated
    from `cable_cell`.
* Create `paintable`, `placeable` and `defaultable` variants that are sum types
  over the respective types that can be painted, placed and defaulted.
* Remove the overloaded `cable_cell::paint`, `cable_cell::place`,
  `cable_cell::set_default` methods to single methods that consume the sum types.

Unit Tests
* Many small changes because many many tests use cable cell API.
* There were no `cable_cell` unit tests! Not such a big deal, since cable_cell
  is tested implicitly in so many other tests
  * but I added cable_cell tests: not much at the moment but will quickly fill
    up as we work on reading, writing, etc.                                                            

Python
* Wrap the new decor type.
* Update cable cell type to reflect read only API.
* Wrap the cv_policy types and implement the same operator overloading.

Fix segfault in modcc when min/max used in net_receive.

* The error occurred when a synapse that use `max` or `min` expressions in the `net_receive` block: we test for linearity to check whether they can be coalesced; we cannot differentiate the expression; an error is raised but not reported; seg fault. 
* Fixes ` as_c_double` printing which does not print integers as doubles as intended. 

Fixes #1275 

Fix a latent bug in the Python wrapper that was triggered in pybind11 v2.6.1

These changes ensure that the GIL is acquired before calling functions in C++ that
may consume a Python object with reference count 1 as an argument, in which case
the object's destructor is called at the end of the function, after any GIL acquired inside
the function would have been released.

Fixes #1271.

* Move PR message instructions to 'Making a PR' section.
* Move coding instructions to 'Write code' section.

* Remove mention of the Copyright Transfer Agreement

* Fix pybind11 issues.

* Retract suspicious fix.

* Address comments.

Replace CMAKE_SOURCE_DIR with PROJECT_SOURCE_DIR to pick up the correct paths when used as a sub-module.

Fixes #1266.

Update CMake configure to look for svgwrite when building docs.

Fixes #1128.

Set up contribution guides.

Add a contribution guide, as well as 
* Added contribution section.
* Moved content here from the wiki regarding (contribution) processes.

Also added some incomplete / placeholder sections that can be improved later.
* How to write an example
* How to add a test
* Coding style guidelines

* An explanation of documentation structure.
* Added HBP/Ebrains to acknowledgements.

Fixes #1257

Fixes a bug caused by swapping the `time` and `time_to` vectors in the `shared_state` and not swapping the corresponding data pointers in the `threshold_watcher` and `mechanism`.
- This fix keeps the swapping optimization, and instead, the `threshold_watcher` and `mechanism` hold pointers to the std::vectors (not the underlying arrays) which are dereferenced before use. 
- The mechanism API functions have been renamed. 
- A unit test is also added that checks the interpolation behavior of the `threshold_watcher`.

Fixes #1251 

* Modify `cell kind` concept docs.
* Add `cell description` concept docs.
* Reorder the docs so that the recipe, cell kind and description, interconnectivity, hardware management, domain decomposition, and simulation are consecutive. Then start the more detailed cable cell docs. 

`pow(double x, int y)` is implemented as of ROCm-3.9 (May have been fixed earlier, don't have access to earlier versions)

Change from long double to double for storing floating point values in modcc.

WSL, and Windows generally, treats long double differently than Linux, which leads to inconsistencies in literal numeric values in code generated by modcc on the two platforms.

Fixes #1245

* Add a readme for the C++ examples
* Improved readme for dryrun

Fixes #868, #1107.

Major changes in Python library and documentation:

* Add global state (immutable after initialization) for the Python module that
  manages the mapping between probe metadata types and the method by which the
  corresponding [sample data can be collected and presented as Python objects.
  This is necessary for decoupling the implementation of the simulation wrapper
  from that of the various cable cell probe types.

* Wrap each of the C++ cable cell probe types with a Python function that
  returns a `probe_info` object.

* Add code for converting and accumulating C++ probe metadata and sample
  data, registered in the module global state against the metadata type info.

* Make the `arbor.simulator` object responsible for recording all spike and
  sample data, with corresponding new methods.

* Use NumPy arrays and structured datatypes for returning spike and
  sample data.

* Place Python schedule proxies under an abstract interface so tha...

* Return `arb::morphology` from `load_swc_xxx` methods for consistency with the NeuroML interface. 
* Fix unit tests.
* Fix documentation

Added a CONTRIBUTION.md page for the project.

Contains a link to the wiki with more detailed description of the contribution workflow.

Fixes #1230

Fixes #1236

Fix Python linking errors on some platforms.

This appears to be related to the GCC visibility of methods overridden in header files, and is fixed by moving the implementation to the corresponding cpp files. 

Addresses the Travis CI failure of #1225

* Add an example describing the content of a recipe in the recipe concepts page in the docs.
* Mention that recipes are necessary, but can be abstracted away using the python `single_cell_model` (Hasn't been documented yet).

Speed up plotting in seaborn.relplot by disabling calculation of confidence intervals

Update the documentation to mention additional packages used in some python examples.

Addresses some of the warnings from static analysis tools in #1084 

* fix some instances where exceptions were constructed, but not thrown
* fix use-after-move in a unit test.

* Adapt to moved readthedocs account

* Various DOC changes/updates, most importantly to:
* Single Cell Model
* Recipe

* Put generated mechanisms for each built-in catalogue in their own namespace.

* Remove blank line from `ball_and_stick.swc` 
* Mention that blank lines in SWC files indicate end of data.
* Check for trailing data after parsing in the python interface.
* Leave stream in good state if `parse_swc` encounters an empty line indicating end of data.

* Rename `as_segment_tree` to `load_swc_arbor`.
* Remove `swc_mode`: `sort_and_validate_swc` now performs the `relaxed` checks by default, `load_swc_arbor` performs the `strict` checks. 
* Change `load_swc_xxx` to only accept `swc_data` as an argument
* Remove default constructor for `swc_data` and enforce validation of SWC records via `sort_and_validate_swc` at construction.
* Add fixes for docs and unit tests.