Support for weighted Wilcoxon test

[LucaCappelletti94]

At this time, the Wilcoxon test accepts two vectors of values. In some instances, such as in the comparison I am running, I am bucketing in buckets of irregular-size samples. Larger buckets need to have a larger impact on the overall outcome of the test, and as such we can include in the input vector also the count associated with the sample.

I will build this feature in a pull request built on top of #9, so @larsgw do check it out when you have time. I am happy to discuss any of it.