Add NominalGEE and OrdinalGEE to api

This PR adds NominalGEE and OrdinalGEE to the API.  Some related enhancements are bundled in too, specifically:

* Privatize the Multinomial and MultinomialLogit classes, make _Multinomial the default family for NominalGEE.  Automate the setting of the number of levels.

* Make binomial the default family for OrdinalGEE.

* Docstring changes related to above.
Is probit a family or a link? 
for GLM probit is a link 
`Binomial` is the only binary family