Feature/rewrite docstrings skll data featureset

[RobertImbrie]

• Add space between parameters.
• Change ` to `` when referring to code.
• Remove 'optional' from parameters with default values.
• Move default values next to parameter names.
• Specify shape of arrays and array-likes.
• Update skll.FeatureSet to skll.data.FeatureSet.

Notes: Sci-Kit Learn's documentation sometimes formats array shapes as: example_array, shape (label1, label2)

Sometimes it formats array shapes as: example_array of shape (label1, label2)

I used the second option because it reduces ambiguity. Otherwise, if a parameter takes multiple data types, it's sometimes unclear which data type has the shape.

[codecov[bot]]

Codecov Report

Merging #685 (afa70fb) into main (8380016) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #685   +/-   ##
=======================================
  Coverage   96.85%   96.85%           
=======================================
  Files          63       63           
  Lines        9098     9098           
=======================================
  Hits         8812     8812           
  Misses        286      286           

Impacted Files	Coverage Δ
skll/data/featureset.py	90.50% <ø> (ø)

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