Decide on whether to save intermediate draws

With the move to posterior_sampling_core we now by default save the MCMC draws every 50 draws into a temporary file, because we by default set posterior_sampler_options.save_tmp_file=1;

I think this is very inefficient and therefore should be 0 by default (same behavior as before the move), with an interface provided to change the option. @rattoma You added this behavior. Was the reason that slice should be treated differently?