Add pytests

[AdamOrmondroyd]

PolyChordLite doesn't currently have any tests besides checking that run_pypolychord.py runs. This PR adds tests using pytest, which are currently:

• Replicating run_pypolychord.py
• The above plus using cube_samples(which will fail with MPI until make_resume_file() incompatible with MPI #101 is complete)
  Both with and without MPI

This framework would allow an array of different likelihoods and settings to be iterated over without needing another run_something.py every time.

[AdamOrmondroyd]

Had to add some more boilerplate to pyproject.toml to get the optional dependencies to work properly