took heatherleaf's suggestion to calculate Nr faster
removed katz cutoff demo since it's no longer implemented by the SimpleGoodTuringProbDist class, cf https://github.com/nltk/nltk/pull/544
fixed probability distribution sum calculation bug in ProbDistI.generate()
added SUM_TO_ONE=False to SimpleGoodTuringProbDist as the sample probabilities do not sum to one