Feature request: add pytest

[richelbilderbeek]

Although MHCnuggets appears awesome, there are no tests to prove that it works correctly. I would enjoy be able to be sure that MHCnuggets at least works on some minimal level, especially in a continuous integration test (stub added with #13).

I suggest to add at least one test, which can be copied from the Jupyter notebook. Apparently, pytest is the recommended Python testing framework, so I suggest to use that one.

I do volunteer to do so, also because I need MHCnuggets for my own research.