Parallel evaluation of tasks

[Neeratyoy]

What does this PR implement/fix? Explain your changes.

Uses joblib to evaluate different folds of a task in parallel, in order to speed-up the purely sequential evaluations of tasks right now.

How should this PR be tested?

The unit tests added to test the accuracy of the distribution of evaluation and collation of results.

[codecov-io]

Codecov Report

Merging #1020 (c2cbfe2) into develop (ab793a6) will increase coverage by 0.13%.
The diff coverage is 84.48%.

@@             Coverage Diff             @@
##           develop    #1020      +/-   ##
===========================================
+ Coverage    88.04%   88.18%   +0.13%     
===========================================
  Files           36       36              
  Lines         4568     4594      +26     
===========================================
+ Hits          4022     4051      +29     
+ Misses         546      543       -3     

Impacted Files	Coverage Δ
openml/runs/functions.py	83.61% <72.72%> (+0.15%)	⬆️
openml/config.py	84.78% <100.00%> (+5.10%)	⬆️
openml/testing.py	84.24% <100.00%> (+0.70%)	⬆️
openml/utils.py	90.66% <0.00%> (-0.67%)	⬇️

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