Fix saving of _mh_mode after integration of SMC

60c0ed01 changed the behavior of compute_mh_covariance_matrix to not save the _mh_mode.mat file. That is now done in compute_posterior_covariance_matrix. This breaks backward compatibility as e.g. fs2000.mod now does not save the _mh_mode.mat anymore via the call to marginal_density.m.