[bdg-utils-13] Added code to implement a poisson mixture model.

Adds code to fit a mixture of poisson random variables. Additionally, adds a few mathematical utils. Closes #13.
Ah, yes. I named it as `aggregateArray` in an earlier version of the code where this was being used as the aggregate function across an RDD of Arrays. I think this code isn't being used anymore (in favor of the Breeze libraries) and should be removed.