Refactor GEE margins, improve test coverage

This refactor does a few things, prompted by #2156:

1. Removes instance methods for mean_deriv and mean_deriv_exog in favor of methods that are overridden for the GEE nominal model.

2. Simplified the derivative calculations for the nominal model so that the scipy sparse matrix library is not needed.

3. Add GEEMargins to the api [Update: we don't need this so I took it out]

4. Add tests against Stata for marginal effects

5. Add a test of the 2-class nominal model against logistic GLM

6. Change the handling of the Poisson scale parameter to agree with GLM (fix scale=1)

7. Add a get_margeff method to GEEResults so we can get marginal effects in the same way that we get them for discrete models.



I think we should account for offset/exposure if they are there, and if the marginal effects are being calculated at model.exog.  I think I have this working now.