Included evaluation of true coverage

For simulated examples the true coverage is evaluated. For all other
datasets create_tabular will show None.
The coverage was reversed to use the prediction (mean under the
posterior predictive). Note, that the "repeated sampling" assumption is
treated indirectly via averaging over the 10 seeds. For simulated data
this works decently, for real data one often (but not always, cf. wine)
sees a discrepancy.