Improve code comments

[anapaulagomes]

This PR is an attempt of making the comments stick to a style. We're considering to adopt Google's docstrings but open to suggestions as usual.

• Add pydocstyle as pre-commit hook
• Get rid of single quote comments
• Fix pydocstyle complains, except for the ones that would demand to add new comments:

D100: Missing docstring in public module
D103: Missing docstring in public function
D101: Missing docstring in public class
D107: Missing docstring in __init__
D102: Missing docstring in public method

[berinhard]

The code looks good for me! I left one small comment on a docstring, but once it's fixed, we can merge this. Do you think we should add black to the project as well?

[anapaulagomes]

I still want to double check this before getting it merged, @berinhard. Thanks for your comments!
About black: I'm 100% in but we should do it in a separated PR.

[berinhard]

Thanks for being careful with the code comments @anapaulagomes! About Black, I've created issue #38 to address that.