Beta Regression in statsmodels

[josef...@gmail.com]

I'm about to merge Beta regression into statsmodels

https://github.com/statsmodels/statsmodels/pull/7543

API is still unstable and needs improvements, but the core results look good.

Here's a gist notebook with some examples

https://gist.github.com/josef-pkt/4d0fd829c8fbc1c4237d989ba3dbb088

The PR's for this started a long time ago by Brent Pederson.

Beta model is part of the new statsmodels.othermod directory,. 

We should get more likelihood models that require two or more link functions, and models for distribution with constrained support like interval or positive data.

Josef