Sample the parameters for the model y ~ N(mu0, sigma^2) with a flat prior on mu0 and sigma ~ Unif(0, A).
sample_params_mean(y, params)The updated named list params with draws from the full conditional distributions
of sigma and coefficients (and updated mu).
n x 1 vector of data
the named list of parameters containing
mu: vector of conditional means (fitted values)
sigma: the conditional standard deviation
coefficients: a named list of parameters that determine mu