return a random number with Psi distribution
psi1()
A numeric value
Christen, J. A. & Fox, C. A general purpose sampling algorithm for continuous distributions (the t-walk). Bayesian Analysis, 2010, 5, 2, 263-281.
Other twalk_auxiliary_function functions: blow_kernel,
hop_kernel,
log_ratio_density_blow,
log_ratio_density_hop,
run_twalk_MCMC,
traverse_kernel, walk_kernel