tidybayes integration

[mjskay]

Saw the announcement of the new release on Twitter, looks awesome!

I had two questions about tidybayes integration:

1. I see you're using tidy_samples instead of implementing a generic for tidybayes::tidy_draws... Are there limitations of the tidy_draws interface that motivate this choice or is there a difference in semantics? If there are changes in tidybayes that would make it possible to make the interfaces consistent I'd be open to it.

2. Any interest in implementing add_fitted_draws/add_predicted_draws/etc? I haven't come up with a good way to make that easy on package developers yet but if there was interest on your part this might be the opportunity to think through how to make that happen. There is also a feature discussion for {posterior} related to generics for these kinds of functions (https://github.com/stan-dev/posterior/issues/39) that I will probably be basing future implementations of add_fitted_draws/add_predicted_draws/etc on, having your input there might be helpful too.