Arbor Library v0.5.2

This release fixes an error in the CI generated Python wheels, which are as of this release available on PyPI. Other than those fixes, this release is identical to v0.5.1.

Since v0.5 there have been some major features, and many small fixes and improvements.

Core API features:

• [C++/Python] Labels instead of indices for placeable item identification.
• [C++/Python] Morphology file format support: Arbor Cable-Cell Format.
• [C++/Python] Morphology file format support: Neurolucida ASCII format.
• [C++/Python] Morphology file format improvements: SWC.
• [C++/Python] Simplified connections and junctions.
• [C++/Python] Enable simulation resume/restart.
• [C++/Python] Add post events functionality to support models with STDP synapses
• [C++/Python] Allow dynamically creating and loading of mechanism catalogue

Documentation:

• Documentation URL changed to docs.arbor-sim.org
• New Python examples and tutorials
• Ever more complete documentation
• Added Code of Conduct

Build / CI:

• Spack distribution
• CI generated binary Python wheels
• Apple M1 compatibility
• CI moved from Travis to Github Actions
• Improved Python and C++ unit testing
• ARM CI
• Sanitizer CI