Filter out stale loglinear option

With a previously set loglinear option, both k_order_pert and
stochastic_solvers at order>1 transformed the first order solution, but
kept the higher order terms untransformed. Now, both functions output an
error.
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.
Merge pull request #391 from JohannesPfeifer/master

Small bugfix in k_order_pert and check for stale order loglinear