Make estimation set the mode-file-option to empty unless specified otherwise

If a mode-file has been specified once, it will always be reloaded in all subsequent calls to estimation, thereby overwriting the MCMC results set with set_all_parameters(xparam,estim_params_,M_) and will conflict with the estimation of other specifications where e.g. the parameters to be estimated have changed.

The check currently always is isempty(options_.mode_file)

Also, setting mode_file=[] creates an error. Thus, the only way of deleting the mode-file options currently is manually setting options_.mode_file=[];