Get finite support

@lileicc @cberzan Here I add the methods getFiniteSupport() to each distribution. This will be used for implementing Gibbs Sampling.
@lileicc Also, it's not true that all the distributions will call getLogProb or getProb during getFiniteSupport (like binomial). So it should be necessary to call CheckHasParams. In order to get a uniform implementation, I suggest that we should call CheckHasParams for every distribution.