function to sample from hyperpriors of a specified probability density function; note that initial values for sigma of lognormal random effects are fixed to a small value (0.05) to prevent numerical errors
switch_sample_prior(pdf, cur.par)
probability density function (pois1, poisson, normal, unif.disc, unif.cont)
a vector giving parameters for the specified distribution; only the first is used for single parameter distributions
a vector of length n samples from the desired distribution