Create interface for specifying filtering transformation of data

Decision at Advanced User Meeting 2023:

varobs(hp_filter=1600) y;
varobs(one_sided_hp_filter=1600) c;
varobs(bandpass_filter=[2 16]) R;

where the three options are the filters currently allowed for stoch_simul. Write the result into 3 by 1 cell array options_.varobs_transformation where column 1 corresponds to options_.varobs_id, column 2 is a string of the filter name, column 3 is the specified parameter value (empty if the default is chosen).