Move filling of decision rules to respective orders

Previously, with pruning set to 1 and the option k_order_solver, the
fields like dr.ghu were not set in k_order_pert, even at order=1. But
stoch_simul tries to use them to display the decision rules. Similarly
for order 2. The respective setting of the decision rule fields is now
moved to the respective cases and always conducted at order 1 and
conducted at order 2. This avoids crashes if decision rules of
theoretical moments are requested.