Fepois: Add support for predict with fixed effects

[s3alfisc]

Context

Currently, the predict method is disabled for Poisson Regression with fixed effects. Unfortunately, I cannot recall why... 😅

To do

• Enable predict() for Poisson regression with fixed effects.
• Clean up ValueError when trying to run Fepois.predict() on a fixed effect model in tests/test_errors.py.
• Add unit tests to tests/test_vs_r::test_single_fit.

[leostimpfle]

While I haven't completed implementation of all separation checks yet, I would like to pre-register my interest for working on this issue 🙂

[s3alfisc]

More than happy to already assign this to you Leo! =)