P values calculation not documented

Despite being the recommended way to compare phenotypic activity/consistensy, the usage of the p-value functions is [not documented](https://github.com/search?q=repo%3Acytomining%2Fcopairs%20p_values&type=code). I think it is worth adding that to the wiki/docs. Let me know where you would like to have it if I figure it out before it is up there, so I can open a PR.