WIP: fit_constrained

add constrained fitting to models

starting with generic setup and Poisson as example.
OLS was test example for transformation, but has separate RestrictedGLS implementation, PR #1673

- needs offset for all models except linear, see #1640
  currently only count models and GLM have offset

- does not return results instance, currently returns params and bse, calculates cov_params
  Those two need to be overwritten in the creation of the results instance.
  I would prefer generic results creation see #1615, but will start with a full test case for Poisson.


code duplication: 
`base.TransformRestriction` has the same purpose as Kerby's version in GEE. I haven't checked if the result is identical.

- API decisions, naming, ...