Adapt evaluate_planner_objective.m to higher order and perfect foresight

Supersedes #564 (closed) and is related to #1678 (closed) See also https://forum.dynare.org/t/evaluate-planner-objective-in-non-linear-model/15492

Remaining:

• fixing or documenting the regression in behavior introduced in e880d1bc. In contrast to previous behavior histval will be ignored. planner_objective_value now returns conditional and unconditional welfare instead of welfare for different values of the Lagrange multiplier. (done in !1923 (merged))
• the question of an interface for setting the initial condition (preprocessor#64 (closed))
• Adding higher order results to evaluate_planner_objective
• Compute unconditional welfare at order>2
• Documenting the recent changes