Porting the CI/CD system to GitHub Actions

[dalonsoa]

PR #855 started to implement the current CI system in GitHub Actions. A couple of things are missing to make it fully functional and be able to replace Travis and Appveyor:

• Solve problem with Sundials libraries in MacOS. They seem to install properly, but tests fail, complaining they are not found.
• Implement a scheduled testing. This should be easy by using on.schedule.
• Implement a CD system to deploy PyBaMM to PyPI after a tagged commit to the master branch. Again, this should be easy using this Action.

[valentinsulzer]

I also had problems with sundials libraries in MacOS. One difference is you need to specify DYLD_LIBRARY_PATH instead of LD_LIBRARY_PATH.

[valentinsulzer]

I think this can be closed? Re-open if not