Add stratified sampling

[vrtsig]

Let me know if this looks useful :-)

• adds stratified (group-by-group) bootstrap sampling in order to improve performance for imbalanced datasets
• requires a small change in the function signature for all bootstrap functions

[coveralls]

Pull Request Test Coverage Report for Build 42

• 64 of 67 (95.52%) changed or added relevant lines in 4 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+1.3%) to 88.037%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
stability_selection/bootstrap.py	22	25	88.0%
<!--	Total:	64	67	95.52%	-->

Totals
Change from base Build 41:	1.3%
Covered Lines:	287
Relevant Lines:	326

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💛 - Coveralls

[thuijskens]

Awesome! Thanks a lot for the contribution 👍