Get finite support

@lileicc @cberzan Here I add the methods getFiniteSupport() to each distribution. This will be used for implementing Gibbs Sampling.
@lileicc Just because I think we should checkHasParams before using getFiniteSupport for every distribution. It's not meaningful to getFiniteSupport if the parameters are not complete. Even though there is no difference for Beta distribution when parameters are different, I think we still need to check it first.