sample_subj_ref: Sample subject random effects
In zhifeiyan/WeibullHM: A Gibbs Sampler of a Bayesian Weibull Hidden Markov Model

Produce a Gibbs sample of subject random effect of a subject in a state

1	sample_subj_ref(y, mu, var_logscale, ref_std, i, s)

`y`	a matrix of current update of Weibull log scale parameters
`mu`	a vector of current update of intercept effects in all states
`var_logscale`	a vector of current update of variances of Weibull log scale parameters in all states
`ref_std`	a vector of current update of standard deviations of subject random effects
`i`	a specific subject
`s`	a specific state