Effective Number of Parameters in kfold

[paul-buerkner]

Summary:

Implement effective number of parameters in kfold.

Description:

@avehtari noted that neither rstanarm nor brms computed the effective number of parameters (2nd row in the loo estimates slot) in kfold. Since we couldn't think of a reason, why we shouldn't compute it, I added it to brms (https://github.com/paul-buerkner/brms/blob/master/R/kfold.R line 260 ff). This implementation could be used to quickly implement it in rstanarm as well.

[jgabry]

Thanks!