Pre-commit hooks to enforce Google-style docstrings

[sadielbartholomew]

Set up two new pre-commit hooks to target the formatting and style of the docstrings, namely:

• the docformatter checker and auto-formatter, notably to wrap summary & description lines that are too long;
• the pydocstyle checker, which checks many more aspects than the above but doesn't provide means to automatically make the changes in-place (hence the decision to use both tools).

Currently the docstrings across our cf tools libraries are all in Google format, so this is the style that we should run with. The only notable aspect of Google docstrings I have observed we aren't particularly consistent with is that:

The docstring should be descriptive-style ("""Fetches rows from a Bigtable.""") rather than imperative-style ("""Fetch rows from a Bigtable."""), except for @property data descriptors, which should use the same style as attributes

So, though the pydocstyle can't detect this, we should try to consistently be "descriptive" not "imperative", my interpretation of that being to say what the object does rather than what we would ask it to do (if it could do anything we said without us having to code it up).

[codecov-io]

Codecov Report

Merging #22 (d826343) into master (f77b966) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #22   +/-   ##
=======================================
  Coverage   78.62%   78.62%           
=======================================
  Files           2        2           
  Lines         720      720           
=======================================
  Hits          566      566           
  Misses        154      154           

Flag	Coverage Δ
unittests	78.62% <ø> (ø)

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Impacted Files	Coverage Δ
cfunits/__init__.py	77.78% <ø> (ø)
cfunits/units.py	78.64% <ø> (ø)

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