ms-sbvar: remove unused field removal statements (and set options_.datafile='' in global_initialization)

Added an option to decide if dsge_likelihood should call univariate filters when the covariance matrix of
the prediction error is singular (default is yes).

options_.gradient method = {12,22} two points formula.
options_.gradient method = {13,23} three points formula.
options_.gradient method = {15,25} four points formula.

Still need to decide what to do with (local) option zgrad.

portfolio models)

all relevant code to stochastic_solvers.m)

Save *all* the prior draws generated by prior_sampler, even if info(1) returned by resol is positive.