Add tests

[Jack-42]

I would highly recommend that we add tests to this project to help verify code-correctness. See:
https://docs.python.org/3/library/unittest.html

There's also pytest:
https://docs.pytest.org/en/stable/

Here's examples of tests defined in some other Cheminformatics libraries:

• DeepChem: https://github.com/deepchem/deepchem/tree/master/deepchem/utils/test
• useful_rdkit_utils: https://github.com/PatWalters/useful_rdkit_utils/tree/master/tests