I don't like adding `if`s to a superclass that checks attributes that are only available in some subclasses.
I thought about just adding the method to the CountModels.

But I want a **generic** solution. Here I don't need any special code for `loglike_method` in NegativeBinomial except registering the keyword.
plus, we will need the same for other things see #1093 (e.g. diagnostic tests)