Perform Diagnostics on CmdStan Fit
Arguments
- fit
 A CmdStanMCMC object returned from cmdstanr::sample()
- plot
 Logical; whether to show diagnostic plots
- observed_y
 Optional vector of observed values if predictive p-value is to be computed, the survey/survival length of each individual
- delta
 Optional vector of censoring indicators (0 = censored, 1 = dead/failed) if predictive p-value is to be computed