ENH: GEE exposure and improvements to from_formula

This PR adds exposure to GEE, which should behave equivalently to exposure in GLM. 

I also added better argument handling in from_formula, so that exposure and/or offset can be passed as variable names or as data arrays.

There are also some new tests for offset, exposure, and predict.

Closes #1991 
These tests don't test the code path with exog is not None.
I'll add a test case in my rebased branch