Efficiently sample multivariate normal using precision matrix from \(x ~ N(Q^{-1}a, Q^{-1})\), where \(Q^{-1}\) is the precision matrix
draw_mnorm(precision_mat, precision_a_vec)precision matrix Q of the multivariate normal distribution
a vector a such that the mean of the multivariate normal distribution is \(Q^{-1}a\)