Check use and precedence of jscale and document it.

The use of bayestopt_.mh_jscale and bayestopt_.jscale as well as options_.mh_jscale is confusing an potentially buggy (see also http://www.dynare.org/phpBB3/viewtopic.php?f=1&t=5072). In dynare_estimation_1 we use

mh_scale_fname = [M_.fname '_optimal_mh_scale_parameter.mat'];
if exist(mh_scale_fname)
    if options_.mode_compute == 0
        tmp = load(mh_scale_fname,'Scale');
        bayestopt_.mh_jscale = tmp.Scale;
        clear tmp;
    else
        % remove the file if mode_compute ~= 0
        delete('mh_scale_fname')
    end
end

to read out the mh_jscale from a previous run of mode_compute=6. But this is after dynare_estimation_init where the mh_jscale seems to be translated into the bayestopt_.jscale actually used in estimation. Thus, it looks as if this statement does nothing.