Discretize Method for Discrete Distributions.

[alanlujan91]

What should the dist.discretize(***) method do for discrete distributions?

My first idea was to return a copy of self, since the distribution is already discrete.

My second idea is that now that DiscreteDistributions and derivatives have the limit attribute, the dist.discretize(***) method could be used to generate a different discretization of the upstream distribution, by changing N, method, endpoints, or any other keyword that generated the discretization.