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.

* Added a `POST_EVENTS` procedure to nmodl, that takes an argument that represents the time since the last spike on the cell. In the event of multiple detectors on the cell, and multiple spikes on the detectors within the same integration period, all spikes will be processed by the synapse. Spikes are processed only once and then cleared.
* Added 3 vectors to the shared state required to dispatch post-events: `cv_to_cell` map, `time_since_spike` holding max(num_detectors) slots per cell, and `src_to_spike` mapping spike sources (detectors) to slots in `time_since_spike`.
* Renamed `vec_ci_` to `vec_di_` (to better reflect that it stands for **domain index**). Named the new `cv_to_cell` index as `vec_ci_` in the mechanisms.
* Fixed existing unit tests and added new tests for the new post-events functionality.

Fixes #1206

First phase of dynamically loaded mechanism catalogues.

- Add functionality to load and use catalogues from DSOs in Python and C++.
- Add documentation and support scripts to build catalogues DSOs.
- Re-factor CMakeLists.txt for mechanisms
- Limitations
  - Needs (compatible, preferably identical) arbor sources when building catalogues
  - Can only work on MacOS and Linux (uses dlfnc)

Ranges (both <,> and FROM/TO) can now be real-valued and negative.

- Allow negative and real numbers  in `<..., ...>` and `FROM ... TO ...`
- Add and extend tests.

* Respect CMAKE_INSTALL_PREFIX in python lib path

* Set `base` and `platbase` to the CMAKE_INSTALL_PREFIX, if defined,
when asking Python for the default python lib install path.

* Differentiate between explicit and default prefix.

* Test for defaulted CMAKE_INSTALL_PREFIX, in which case, use the system
python site packages path, so that it's overall less excitingly awful in
python virtual environments. If you want /usr/local, set it.

* Used Sphinx glossary directive to create terms that can be referred to in `concepts/morphology.rst` and `concepts/labels.rst`.
* Added note box on difference with Neuron, add source for NEURON cylinder discretisation
* Parameters added to svg generation script: turn off colors, root
* Fix cpp TOC duplication, match ordering in other sections.
* State default cv policy
* center/re -> midpoints
* Clarify and remove compartment term

Introduce two minor changes to the Python API
to handle mechanism_catalogues idiomatically.

Instead of

import arbor as A

cat = A.default_catalogue()

if cat.has('hh'):
  print("Found HH.")

for mech in cat.keys():
   print("*", mech)

we can now write

import arbor as A

cat = A.default_catalogue()

if 'hh' in cat:
  print("Found HH.")

for mech in cat:
   print("*", mech)

which is closer to the expectations of Python users.

* Add docs tutorial page for single_cell_recipe.py example
* Adapted single_cell_recipe.py slightly
* Improved labels and fixed some broken cross-linking
* Fixed a typo here and there

* Set VERSION to 0.5.1-dev

* Add link to Zenodo entry for version 0.5 release.

Numpy is now explicitly required for use in the Python probes interface.

Fix some broken hyperlinks in the rst documentation.

* Add the ability to use json and pybind11 libraries installed on the system instead of the
  vesrsions of those libraries bundled as submodules in the Arbor repository
  * Turned on by default
  * Toggled using the new `ARB_USE_BUNDLED_LIBS` CMake flag
* Update documentation for installation
* Update CI and pip workflows to opt in to the bundled libraries

* Add unit test that checks the expected contribution of an
annulus to the embed_pwlin integrated area, when a zero-length segment
with differing radii exists in the segment tree.

Revert a change introduced in #1282 that required the python module for arbor to build the documentation.

This breaks the readthedocs build, where arbor is not available.

* Remove arbor dependence from `doc/conf.py`, reverting to the original "out of core" approach
* Fix other recently introduced issues that lead to CMake errors when configuring the documentation
  when the Python target is not configured.
  * The CMake variable `PYTHON_EXECUTABLE` wasn't set, but was required to make calls to the
     Python interpreter.
  * Instead of relying on pybind11 to find Python, explicitly detect it using `find_package(Python3 ...)`
  * Make Python3 a hard requirement when building the Python wrapper
  * And a soft requirement otherwise, that will fail only when the docs are built.

Update the PR licensing rules in the PR contribution guide.

Fix CMAKE to use PYTHON_EXECUTABLE that corresponds to the version of Python used to build the python module.

* Add 2 new examples to `python/examples` and their corresponding tutorials in the docs. 
    1. `single_cell_detailed.py` : a slightly more complex example using `single_cell_model`
    2. `single_cell_detailed_recipe.py`: equivalent to `single_cell_detailed.py` but using a recipe. 
* Change `load_swc`-> `load_swc_arbor` for consistency.
* Run Sphinx with specified PYTHON_EXECUTABLE.
* Set PYTHONPATH env var in doc/CMakeLists.txt, such that it propagates correctly, including locally.
* Build pyarb BEFORE html such that docs build can import arbor in Github Actions script.

Add API for `single_cell_model` and `label_dict`