Add ar-option to estimation command
Currently, the ar-option is only allowed in stoch_simul. Its value then also tranfers to the computation of moments in compute_moments_varendo.m. We should explicitly implement ar in estimation. If specified, it should also trigger the moments_varendo option. This avoids setting options_.ar before the estimation command. Maybe we should split options_.ar into options_.ar for stoch_simul and options_.var_endo_ar for estimation in order for one command not to affect the computations from the other one as is currently the case.