Add testing for simple and/or common use cases

[Liam-Deacon]

Currently the only testing that is done is to check that the phaseshifts.lib.libphsh compiled fortran extension is importable, however no tests are done to check the numerical accuracy of the results, nor the behaviour in different scenarios (e.g. checking against various compiler warnings such as #6).

Furthermore no testing is done on the phsh.py script which provides a more user friendly wrapper to run the entire Van Hove program suite to generate the end phaseshifts.

As such tests are needed for the following:

• Unit tests for key fortran functions (exposed via phaseshifts.lib.libphsh) as well as essential python functionality
• Behaviour tests for known model inputs against expected outputs and test things such as concurrent use
• Integration tests for phsh.py