Add Kruskal-Wallis ranking and Kruskal-Wallis Test
`kruskalWallis` now ranks samples
Add `kruskalWallisTest`
Add more Kruskal-Wallis tests
Add documentation for Kruskal-Wallis module
Small changes

 - Inline square function
 - Improve readability
Improve readability a somewhat

N.B. current implementation of mean uses Welford algorithm which
is less numerically robust than KBN summation it's more readable
Add module header
Merge branch 'kruskal-wallis'