Add check for docstring convention (PEP 257)

[sbellem]

What is this? Python has a style guide for docstrings in PEP 257. pydocstyle is a tool to check the docstrings according to the conventions in PEP 257.

The configuration is in setup.cfg under the [pydocstyle] section:

[pydocstyle]
convention=pep257
add_ignore=D100,D101,D102,D103,D104,D105,D107,D200,D202,D205,D208,D210,D400,D401,D402,D403

A complete list of error codes is at http://www.pydocstyle.org/en/5.0.2/error_codes.html.

Goal and direction

The goal of this pull request is to introduce the usage of pydocstyle to check docstrings but meanwhile tolerating a lot of "errors" with the intention of gradually fixing these errors and eventually requiring the numpy style of docstrings.

[codecov[bot]]

Codecov Report

Merging #434 into dev will decrease coverage by 0.07162%. The diff coverage is n/a.

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- Coverage   77.31423%   77.24261%   -0.07162%     
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  Branches         856         856                 
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- Hits            4318        4314          -4     
- Misses          1094        1096          +2     
- Partials         173         175          +2