grst/wilcoxon test

[grst]

• (wip): wilcoxon test
• (feat): finish wilcoxon
• (chore): add note about design
• (fix): sparse check
• (chore): update comment
• (chore): add pass to fit
• (chore): more helpful error
• (refactor): alternative_hypothesis kwarg
• (chore): pre-commit hooks
• (fix): misspelling
• (refactor): create base method for non-parameteric classes + wilcoxon test
• add minimal test dataset

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[grst]

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