Allow CV in all posterior predictive checks

[paul-buerkner]

Right now, CV is only supported in selected PP-checks, directly built in bayesplot, e.g. ppc_loo_pit_overlay. But it would be nice to support this in all PP-checks types. For CV based on importance sampling, we can use important resampling of the predictions, for exact CV we immediately have the CV predictions available.

Tagging @avehtari in case he wants to add something.

[paul-buerkner]

@avehtari did I see it correctly, that this is currently being addressed already from the bayesplot side?

[avehtari]

I don't think so