Discuss and document the load_mh_file option

The current behavior of the load_mh_file-option with it recomputing the mode, the Hessian, and the scale-factor by default seems counterintuitive. Given the fixed seed, one should get the same results, but there is the risk of them changing between the reloaded chain and the new elements to be added and thus having a chain with difffering proposal densities. We should at least document this behavior and potentially reload the mode-file by default. See also http://www.dynare.org/phpBB3/viewtopic.php?f=1&t=5051