Kruskal-Wallis Test addition

There are some limitations to highlight:

- No precise calculation of critical value for significance, only approximation using *Chi-Squared* distribution. This means if any sample is smaller than 5 it returns `Nothing`.

- The structure to represent the samples is a `list`. It may be problematic when there are several samples. Would it be worth to add `repa` as a dependency in order to handle multi-dimensional arrays properly? What else could be done?

- I haven't done any performance testing or looked actively for memory leaks. It should be OK for few samples but I can work on that if you think it's necessary.