Docs to numpydoc convention

[ColCarroll]

Right now, running

$ python -m pydocstyle --convention=numpy pymc/ | wc -l
3090

Shows 3090.

It would be a bit much to try to fix all this at once, but a helpful contribution would be to:

1. fix one directory at a time: use python -m pydocstyle --convention=numpy pymc3/path/to/directory/ and do what the suggestions say
2. add python -m pydocstyle --convention=numpy pymc3/path/to/directory/ to pymc3/scripts/lint.sh
3. make a PR
4. repeat.

[eigenfoo]

I think the reason this issue has been so persistent is because there are quite a few D10X: Missing docstring in X function errors. This is pretty bad because missing docstrings should definitely be failing some test, so should we could agree to fix up the stylistic issues, and allow missing docstrings to fail the lint build?

[michaelosthege]

When the roadmap is clear, I'd like to help with this one.

[ricardoV94]

@OriolAbril Is this issue still relevant?

[OriolAbril]

probably yes, hopefully we don't have that many errors now though.

[ricardoV94]

Closing in favor of #6378