Added option nonlinear_filter_initialization.
Default value is 1 (initialization with the ergodic variance of the reduced form solution of the model approximated at order one). If the model has unit roots, the user must use `nonlinear_filter_initialization=3`, which select an identity matrix for the initial covariance matrix of the state variables. A side effect of this option is to temporarily change the value of options_.qz_criterium to a value above one (ie 1+1e-6) so that the unit roots are not rejected. If the model has unit roots and if the and if the option nonlinear_filter_initialization has a value less than 3, the evaluation of the likelihood will fail, because by default the unit root is counted as an unstable root.