Truncated exponential family distributions

[ismailsenoz]

It would be very nice to extend the current functionality to truncated distributions. I already started a proof of concept in truncate branch. This proof of concept covers all the univariate distributions in the repo. Nevertheless, it doesn't have the functionalities for multivariate and matrixvariate distributions.

[ismailsenoz]

@wouterwln @Nimrais @ThijsvdLaar @HoangMHNguyen I have assigned you folks as you all have knowledge with truncated distributions. A joint effort would be nice I think. It is not an urgent issue however a very useful one for extending model classes.