Add check for purely backwards model to k_order_pert

The k_order_solver does not solve purely backwards models (as is the case in stochastic_solvers.m). But in contrast to stochastic_solvers, there is no check and no warning. Entering the mex-file then results in a crash of Matlab
Merge pull request #428 from JohannesPfeifer/k_order_backward

Add check for purely backwards model to k_order_pert