use multiprocessing in validation and analysis for speedup

[AurelienJaquier]

Validation running time went from 100s to 20s on my machine (10s for threshold computation and 10s for validation)

Each validation now runs in parallel in a separate pool.
Each simulation in iv_curve and fi_curve also run in parallel in a separate pool

[codecov]

Codecov Report

Attention: Patch coverage is 93.87755% with 3 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
tests/test_utils.py	90.90%	2 Missing ⚠️
tests/test_analysis/test_analysis.py	75.00%	1 Missing ⚠️

Files with missing lines	Coverage Δ
bluecellulab/analysis/analysis.py	31.60% <100.00%> (-2.49%)	⬇️
bluecellulab/utils.py	97.67% <100.00%> (+1.00%)	⬆️
tests/test_analysis/test_analysis.py	94.02% <75.00%> (-4.48%)	⬇️
tests/test_utils.py	96.36% <90.90%> (-3.64%)	⬇️

... and 1 file with indirect coverage changes

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[AurelienJaquier]

pinging @pgetta since he recommended to parallelize validation for speedup.

[ilkilic]

Awesome! Thanks a lot for the boost!

[pgetta]

Thank you, @AurelienJaquier, that is awesome.