Add Kruskal-Wallis and Friedman Tests for non-parametric data

[ismael-mendes]

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Issue description

When working with three or more groups it's necessary to test whether samples originate from the same distribution (if independents) or detect differences in treatments across multiple test attempts (if dependents). So, I can't find the Kruskal–Wallis Test and the Friedman Test on Accord.Statistics.Testing. I think it's necessary implement these tests to perform that.