TST/ENH: support for formulas in zero inflated models

[josef-pkt]

question on mailing list

It looks like there are no unit tests and examples for from_formula for ZIP or ZINB.

Usage should be
ZeroInflatedPoisson.from_formula("count ~ bin + animal", my_dataframe)

with keyword options for
exog_infl=None, offset=None, inflation='logit', exposure=None

In my main notebook I used only numpy directly
https://gist.github.com/josef-pkt/c932904296270d75366a24ee92a4eb2f

ZeroInflated models are also not in formula.api

[josef-pkt]

I tried a simple example for ZIP from_formula and it works correctly.
I had made a comment in the PR #3908 (comment)

needs unit tests

There is no formula support for exog_infl, see #3903

[josef-pkt]

related issue: if we have formula in creating the model, then we need also formula support for transform in predict and related methods like get_distribution.