Added Distribution to public exports

[AKuederle]

No description provided.

[jmschrei]

I'll need to think about this for a little bit. I use the underscore to indicate a pseudo-private method/class in the sense that the method is not meant to be directly used itself but, rather, is used internally. So, the Distribution object is not meant to be used by itself in the same way that one would use a Normal distribution. If you want to define a new distribution you'd inherit from it in the same way you'd have to implement your own pseudo-private methods.

[jmschrei]

I'm going to close this for now. Thanks for looking closely at the code and making this suggestion, though. I'll keep it in mind and if it makes sense in the future I'll add it in, though!