Add prior values to Stan input for baggr
set_prior_val(target, name, prior, p = 1, to_array = FALSE)list object (Stan input) to which prior will be added
prior name, like hypermean, hypersd, hypercor
one of prior distributions allowed by baggr like normal
number of repeats of the prior, i.e. when P i.i.d. priors are set for P dimensional parameter as in "mu & tau" type of model
for some models where p may be larger than 1, Stan will expect
an array instead of a numeric (even when p == 1), so for compatibiliy
we return fam as an array type