Potentially change use of options_.drop in simulations

Currently, options_.drop is used for both IRFs and moments. However, the simulated series generated with simult_ do not drop periods. I think that this is not desirable and we should also drop periods at the beginning unless the user wants to start at the deterministic steady state.

For linear models, this would not matter. But for models solved at higher order, we would then not knowingly output simulated series that for sure have not yet converged to the ergodic distribution.