supporting offset in poisson

[apoorvalal]

this gives the gist; having an offset argument would involve constraining the coefficient to 1 and being able to model rates (in contrast to the regular use to model counts). Statsmodels supports it under an exposure arg.

more bg

[s3alfisc]

Yes, nice one. Have never used an offset before in my life, so hadn't bothered with this yet 😀 looks like it might not be too hard to add! fixest::feglm() has an "offset" argument, so we should maybe stick with that instead of statsmodels kwarg solution?