reloo generic #55

@paul-buerkner I'd like to add a generic for reloo so we can have reloo.stanreg and reloo.brmsfit methods. But I see that currently brms takes the first argument of reloo to be the loo object. So it can't change to a brmsfit method without breaking backwards compatibility, right? If that's the case then maybe we should use a different name than reloo for the generic and new methods and you can deprecate reloo it in brms. What do you think? Definitely open to other ideas.