Improve docstrings & type hints (Part 4)

[desilinguist]

• chore: ignore E501 errors in various files.
• chore: fix docstrings and types in skll.experiments and skll.learner modules.

To review this, please look at the built documentation and check that under "API documentation", look at the sections "metrics Package" and "utils Package" and confirm that:

• All parameters have a docstring.
• All type hints have clickable types or type hints.
• The default values are all specified as default = blah.
• All other changes look okay.

This is the final PR for #647. Closes #647.

[codecov]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (6409e06) 95.19% compared to head (89b4543) 95.19%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #740   +/-   ##
=======================================
  Coverage   95.19%   95.19%           
=======================================
  Files          29       29           
  Lines        3535     3535           
=======================================
  Hits         3365     3365           
  Misses        170      170           

Impacted Files	Coverage Δ
skll/learner/__init__.py	97.18% <ø> (ø)
skll/metrics.py	97.08% <ø> (ø)
...utils/commandline/compute_eval_from_predictions.py	97.50% <ø> (ø)
skll/utils/commandline/filter_features.py	98.50% <ø> (ø)
skll/utils/commandline/generate_predictions.py	98.68% <ø> (ø)
skll/utils/commandline/join_features.py	98.18% <ø> (ø)
skll/utils/commandline/plot_learning_curves.py	96.42% <ø> (ø)
skll/utils/commandline/print_model_weights.py	95.00% <ø> (ø)
skll/utils/commandline/run_experiment.py	96.87% <ø> (ø)
skll/utils/commandline/skll_convert.py	93.15% <ø> (ø)
... and 2 more

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