Add pydocstyle to CI

[bschroeter]

I am finding pydocstyle to be particularly useful in ensuring that classes/methods etc are properly documented.

There are a bunch of rules that are more semantic than useful and are often conflicting, but the following command is at least ensuring that I have documented every class and method with arguments etc.

pydocstyle --ignore=D107,D213,D203,D413,D105 .

We should add this to the CI pipeline, to ensure that we are staying on top of doc strings in the code, from which automated API docs can be generated.