WIP: Noncentral hypergeometric distribution

There are two noncentral hypergeometric distributions (Fisher and
Wallenius). Fisher's distribution is used for tests based on contigency
tables as the Fisher exact test (necessary for e.g. computing confidence
intervals).

TODO:
* Fisher's NonCHyp
 - tests & doc
 - sampling
* Wallenius' NonCHyp
What's the difference between `logP` and `logpdf`?