Convert docstrings to numpydoc convention.

[tsalo]

Closes None.

Changes proposed:

• Convert docstrings to numpydoc convention.
• Add a linting GHA.

[codecov-commenter]

Codecov Report

Merging #72 (6d2b9b5) into master (2858c48) will increase coverage by 0.01%.
The diff coverage is 50.00%.

@@            Coverage Diff             @@
##           master      #72      +/-   ##
==========================================
+ Coverage   88.61%   88.63%   +0.01%     
==========================================
  Files          10       10              
  Lines         773      774       +1     
==========================================
+ Hits          685      686       +1     
  Misses         88       88              

Impacted Files	Coverage Δ
pymare/core.py	90.90% <ø> (ø)
pymare/effectsize/__init__.py	100.00% <ø> (ø)
pymare/effectsize/base.py	95.65% <ø> (ø)
pymare/effectsize/expressions.py	100.00% <ø> (ø)
pymare/estimators/__init__.py	100.00% <ø> (ø)
pymare/estimators/combination.py	92.00% <ø> (ø)
pymare/estimators/estimators.py	83.65% <ø> (ø)
pymare/stats.py	92.30% <ø> (ø)
pymare/results.py	81.81% <33.33%> (ø)
pymare/__init__.py	100.00% <100.00%> (ø)

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