Use 5 folds for grid search

[desilinguist]

This PR closes #487.

See issue for motivation to change the default number of folds for grid search cross-validation. One by-product of the change is that experiments will certainly take longer to run but they are likely to be more reliable.

Changes:

• Change default grid search folds from 3 to 5.
• Update grid_search_folds documentation
• Change default value of MAX_SKLL_CONCURRENT_PROCESSES to 5.
• Change tests to use 3 grid search folds otherwise tests take too long. Note that the the config parsing tests in test_input.py were correctly modified to check that the default value for grid search folds is 5.

[codecov[bot]]

Codecov Report

Merging #667 (ea4966d) into main (dc5ed05) will not change coverage. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #667   +/-   ##
=======================================
  Coverage   96.76%   96.76%           
=======================================
  Files          63       63           
  Lines        9077     9077           
=======================================
  Hits         8783     8783           
  Misses        294      294           

Impacted Files	Coverage Δ
skll/config/__init__.py	95.11% <ø> (ø)
skll/learner/__init__.py	97.09% <ø> (ø)
skll/learner/voting.py	98.42% <ø> (ø)
tests/test_input.py	99.79% <ø> (ø)
tests/test_output.py	100.00% <ø> (ø)
tests/test_voting_learners_api_1.py	100.00% <ø> (ø)
tests/test_voting_learners_api_2.py	100.00% <ø> (ø)
tests/test_voting_learners_api_3.py	100.00% <ø> (ø)
tests/test_voting_learners_api_5.py	100.00% <ø> (ø)
tests/test_voting_learners_expts_1.py	98.38% <ø> (ø)
... and 6 more

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[desilinguist]

@mulhod it'd be really great if you could test this with gridmap as well. Thanks!