Poisson naive Bayes classifier (PoissonNB)

Added PoissonNB class to naive_bayes.py to implement a Poisson naieve Bayes classifier.  Poisson classifiers have important applications in systems neuroscience.  See: Ma et Al. *Bayesian inference with probabilistic population codes*  Nat. Neuroscience 9:1432 (2006).

why is this file here?

same for out.txt

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