Implement general discrete real distribution (like nominal)

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In the syntax " x >> dict", we should check whether the keys of the dictionary are numeric or symbolic to determine which constructor to call.

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Follow the template from https://github.com/scipy/scipy/blob/v0.15.1/scipy/stats/_discrete_distns.py

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Specifically, we should expose the constructor: rv_discrete(values=((1,2), (.3,.7)))