Allow full parallel evaluation of a task

[mfeurer]

By using joblib.Parallel over the folds/repeats etc. of a task.

[codecov-io]

Codecov Report

Merging #880 into develop will increase coverage by 0.1%.
The diff coverage is 91.48%.

@@            Coverage Diff             @@
##           develop     #880     +/-   ##
==========================================
+ Coverage    88.49%   88.59%   +0.1%     
==========================================
  Files           37       37             
  Lines         4304     4333     +29     
==========================================
+ Hits          3809     3839     +30     
+ Misses         495      494      -1

Impacted Files	Coverage Δ
openml/config.py	95.83% <100%> (+5.26%)	⬆️
openml/runs/functions.py	83.15% <76.66%> (+0.83%)	⬆️
openml/tasks/split.py	94.79% <97.36%> (+0.16%)	⬆️
openml/_api_calls.py	83.14% <0%> (-4.5%)	⬇️
openml/extensions/sklearn/extension.py	91.19% <0%> (-0.13%)	⬇️

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[mfeurer]

Superseded by #1020