repackaging of Fishtest's SPRT calculation code

[jdart]

I have created a separate repository containing just the "stats" directory from Fishtest, as a single Python 3 package. It is self-contained. See:

https://github.com/jdart1/stats

This implements the pentanomial SPRT model. The original code is from Michel Van den Bergh and I have only made small changes so it can exist as a stand-alone module. The original fishtest code has no explicit license and neither does this repo, currently.

--Jon