Allow specifying custom seed for cross-validation

[desilinguist]

This PR closes #593.

• Add a new cv_seed keyword argument to the Learner.cross_validate() method with the same default value as before.
• Create a new cv_seed configuration file field in the “Input” section. Also, show the seed in the experiment log.
• Update the documentation to describe this new field.
• Add a new input test for "cv_seed" field.
• Add new CV test for the “cv_seed” field
  • Add a new test generator that sets a custom seed and then checks whether the folds generated from inside SKLL match expected folds, for both classifiers and regressors. This also needs two new configuration file templates.
  • This test requires computing the expected folds which we were already doing for another, already existing test. So, I factored that code out into a utility function in tests/utils.py.
• Allow cv_seed for voting learners too.
  • Add tests for the voting learners side in the same vein as the regular learners using the same utility function.
• Tweak various existing tests due to this extra field.
• Tweak CI plans for timing
  • test_voting_learners_api_5.py now takes 2x longer due to the addition of a new boolean field so let's move it to a different test partition which is much shorter than others.

[codecov[bot]]

Codecov Report

Merging #707 (a0dabd6) into main (bfe89bc) will increase coverage by 0.03%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #707      +/-   ##
==========================================
+ Coverage   96.89%   96.92%   +0.03%     
==========================================
  Files          63       63              
  Lines        9199     9263      +64     
==========================================
+ Hits         8913     8978      +65     
+ Misses        286      285       -1     

Impacted Files	Coverage Δ
tests/test_voting_learners_expts_1.py	98.38% <ø> (ø)
tests/test_voting_learners_expts_2.py	98.82% <ø> (ø)
tests/test_voting_learners_expts_3.py	98.82% <ø> (ø)
tests/test_voting_learners_expts_4.py	98.76% <ø> (ø)
tests/test_voting_learners_expts_5.py	98.48% <ø> (ø)
skll/config/__init__.py	95.14% <100.00%> (+0.01%)	:arrow_up:
skll/experiments/__init__.py	95.17% <100.00%> (+0.02%)	:arrow_up:
skll/learner/__init__.py	97.13% <100.00%> (ø)
skll/learner/voting.py	98.45% <100.00%> (ø)
tests/test_cv.py	100.00% <100.00%> (ø)
... and 4 more

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[desilinguist]

@Frost45 please take a look at the changes!