Adds priors to a 'JAGS' syntax.
JAGS_add_priors(syntax, prior_list)JAGS_add_priors returns a JAGS syntax.
Syntax containing moment, inverse-moment, or inverse-gamma priors
(dbt_invgamma) uses the BayesTools JAGS module. JAGS_fit()
loads this module automatically;
users running syntax returned by JAGS_add_priors() directly in JAGS
need to load the module first.