A function specific to metab_bayes models. Returns an MCMC object of class
stanfit (rstan::stanfit), which is saved in the metab_model by
default because you should almost always inspect it; see keep_mcmcs
argument to specs() for options for saving space. The rstan methods
for rstan::stanfit objects include summary(), get_stancode(),
stan_dens(), stan_diag(), and many more. See
?'rstan-plotting-functions', rstan::stanfit and the
rstan manual.