Check correctness of optimal policy with linear model

stoch_simul locally sets

if options_.order~=1 && M_.hessian_eq_zero options_.order = 1;

However, M_.hessian_eq_zero only seems to apply to the original model, not the nonlinear one resulting from e.g. ramsey_model.

On top of that, the change is local, so evaluate_planner_objective will crash, because it expects order=2.

The same will happen when declaring model(linear). Note that an additional check is in stochastic_solvers.