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%)	⬇️

---

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 69d443f...08145fc. Read the comment docs.

[mfeurer]

Superseded by #1020