Fix type hints numpy, pandas, and matplotlib

[redeboer]

Type hints for numpy are currently not checked with mypy. This can be done by installing with data-science-types. I have the impression that we use numpy in the wrong way here and there (for instance, numpy.size used here seems to be deprecated), so those issues need to be fixed.

In a follow-up (#131), the shapes of the ndarrays can then be specified in type hints as well.

[redeboer]

I've given this data-science-types a try, but there seem to be some serious issues still (the package has only been around since this year). It's a pity, because the current type hints of numpy.ndarray are just ignored by mypy, but I don't see a way around this for now.

Therefore, I'll put this one and #131 in the icebox for now. It seems that typing for nympy is still under development (both by external packages and by numpy itself).

[redeboer]

This issue has become less relevant since #345. Closing it for now.