Add Hartigan's dip test

[karoliskoncevicius]

Implement dip test. The original R implementation is available in the diptest library.

[Close-your-eyes]

FYI, I think this implementation of diptest has a limitation of 72000 observations per group or so.
Alternatives may be silverman test (https://github.com/jenzopr/silvermantest) or multimode (https://cran.r-project.org/web/packages/multimode/multimode.pdf).

Currently, I have used/tested dip.test though from the diptest package.

[karoliskoncevicius]

Thanks! Very useful information for the future.