Add plot_ppc_pit

[aloctavodia]

Similar to https://python.arviz.org/en/stable/examples/plot_bpv.html is should only handle the logic of u-value.
Once we have LOO methods in arviz-stats, this should also handle plot_loo_pit. Maybe even default to use loo if log_likelihood group is present and not use it, but emit a warning if not present and the argument "loo" is set to true...

[aloctavodia]

plot_pit was added in #159 (still no LOO)