Update E_loo khat diagnostic

[avehtari]

Update E_loo khat diagnostic

[avehtari]

Currently, loo suggests posterior. if I would use posterior to fix this issue, would it stay as suggested?

[jgabry]

It’s possible to keep it in Suggests, check for posterior when the user runs E_loo, and error if it’s not found. But if we think E_loo is an important function and there’s no option to run E_loo without posterior, we should probably move it to Imports instead of Suggests, which will automatically install posterior when loo is installed. The downside is that it forces the user to have all the additional dependencies that posterior requires in order to use anything in the loo package, even the functions that don’t need posterior.

[avehtari]

Importing posterior would add dependency on methods, abind, rlang, tibble, vctrs, tensorA, pillar, distributional
rstanarm, cmdstanr, and brms already import posterior
rstan imports methods, but no posterior, but I think it would be good if rstan would start using posterior, too
other packages depending on loo, depend or import rstanarm, cmdstanr, brms, or rstan, so some packages would get the extra dependencies

I will first make a PR with dependency on posterior, and if decided that no dependency, then I'll copy the functions