[JOSS] Report (and improve) test coverage

Running python -m pytest tests/ --cov=cellpylib (with pytest-cov package installed) currently yields the following coverage report:

---------- coverage: platform darwin, python 3.9.6-final-0 -----------
Name                          Stmts   Miss  Cover
-------------------------------------------------
cellpylib/__init__.py             9      0   100%
cellpylib/apen.py                17      1    94%
cellpylib/bien.py                45      0   100%
cellpylib/ca_functions.py       133     17    87%
cellpylib/ca_functions2d.py     119     69    42%
cellpylib/ctrbl_rule.py          27      0   100%
cellpylib/entropy.py             35      1    97%
cellpylib/hopfield_net.py        28      0   100%
cellpylib/langtons_loop.py      121    116     4%
cellpylib/rule_tables.py         74      6    92%
-------------------------------------------------
TOTAL                           608    210    65%

Wide parts of the library seem to be well-covered and most of the missing statements come from the (rather complicated) langtons_loop module. However, I would expect a higher test coverage in the two ca_functions modules, given that they are the core part of the library.

@lantunes Have you checked which of the statements are currently untested and would you consider expanding the corresponding tests to cover wider parts of the core library?

[lantunes]

I have opened a draft PR here: https://github.com/lantunes/cellpylib/pull/23.

[lantunes]

For reference, the relevant commits are 6fe535171dc5d587b7b90cd1503f3bb64ce0ff78 and 8a950ba832b22d62c632abc2ce3b5b192c1e3dfb.