function to calculate posterior predictive loss given the output object from hierarchicalDS
post_loss(Out, burnin = 0)
Output object from running hierarchicalDS
Any additional #'s of values from beginning of chain to discard before calculating PPL statistic (default is 0)
A matrix with posterior variance (P), sums of squares (G) for the posterior mean and median predictions (compared to Observations), and total posterior loss (D)