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

matlab/k_order_pert.m

@@ -30,6 +30,10 @@ nspred = M.nspred;
 if order>1 && options.loglinear 
    error('The loglinear-option currently only works at order 1') 
 end
+if M.maximum_endo_lead == 0 && order>1
+  error(['2nd and 3rd order approximation not implemented for purely ' ...
+       'backward models'])
+end
 
 switch(order)
   case 1