Mann-Whitney U test should return p-value.

[GregorySchwartz]

I noticed that while it is possible to get the p-value from some tests, many do not have this option. For instance, mannWhitneyUtest. One issue I believe is the TestResult type which over simplifies the result of a test. Instead of a binary type, this type should contain the p-value, critical values, and maybe even sample sizes, but definitely a p-value.

[Shimuuar]

Since 0.11 all statistical tests except Mann-Whitney U test do return significance. And it doesn't because current implementation only calculate whether test is significant or not. So new code for calculation of significance is required.

I renamed this issue to be more specific and mark it as "help neede" since I won't have time to fix it.