Provide interface to access evaluate_planner_objective.m

As noted in #564 (comment 9765), ramsey_policy only allows for order=1. The solution is to replace it by

ramsey_model;
stoch_simul(order=2)

But the downside of this approach is that we do not compute the welfare function in this case and one cannot add it to the model-block. For that reason, we need to provide the user with an interface to call evaluate_planner_objective