Simplify parameter grids

[desilinguist]

This PR closes #618.

• Change the default parameter grids to be plain dictionaries instead of lists with a single dictionary.
• Update docstrings and documentation to make it more consistent and compatible with this change.
• Update tests.

[codecov]

Codecov Report

Merging #619 into master will increase coverage by 3.43%.
The diff coverage is 100.00%.

@@ Coverage Diff @@ ## master #619 +/- ## ========================================== + Coverage 91.71% 95.15% +3.43%  ========================================== Files 26 26 Lines 3031 3031 ========================================== + Hits 2780 2884 +104  + Misses 251 147 -104 

Impacted Files	Coverage Δ
skll/config/__init__.py	95.09% <ø> (ø)
skll/utils/constants.py	100.00% <ø> (ø)
skll/learner/__init__.py	96.11% <100.00%> (+6.37%)	⬆️
skll/metrics.py	97.87% <0.00%> (+2.12%)	⬆️
skll/experiments/__init__.py	95.19% <0.00%> (+6.60%)	⬆️
skll/experiments/output.py	97.36% <0.00%> (+6.84%)	⬆️
skll/config/utils.py	95.65% <0.00%> (+8.69%)	⬆️
skll/learner/utils.py	95.88% <0.00%> (+12.94%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 6bf31ab...5d2b906. Read the comment docs.

[ghost]

It looks good. Can we also have a test case with empty dictionary parameter grid.

[desilinguist]

Almost all of our tests in test_classification.py and test_regression.py are with empty parameter grids since we do not specify an explicit parameter grid either via the API or via the config file. So that's already covered.