RFC: Mann Whitney U test (Wilcoxon rank sum test)

[zross]

Checklist

• [x] Read and understood the Code of Conduct.
• [x] Searched for existing issues and pull requests.
• [x] The issue name begins with RFC:.

Description

Perhaps it's possible to include the Mann-Whitney U test. I'm seeing the Kruskal test which is close!

For reference, in R this is:

wilcox.test(var~grp, paired = FALSE)

[zross]

A possible js implementation: https://gist.github.com/gungorbudak/1c3989cc26b9567c6e50