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.
we also have lower case `gee` as alias to `GEE.from_formula` in statsmodels.formula.api
To follow the pattern of other models, we should also document 

```
import statsmodels.formula.api as smf
smf.gee(...)
```
ordinal_gee and nominal_gee  (underline or not ?) should be added to `formula.api` also