Filter out cases where invalid starting value leads to NaN or Inf in Jacobian.

Replaces cryptic message "Some element of Newton direction isn't finite. Jacobian maybe singular or there is a problem with initial values" with more expressive message.
Merge pull request #427 from JohannesPfeifer/dynare_solve

Filter out cases where invalid starting value leads to NaN or Inf in Jac...
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
Cosmetic changes.
Fixed bug.
Fixed bug related to the shape of the weights vector in paricle filter with pruning.
Added test on the reciprocical condition number of PredictedObservedVariance.
Stop the simplex algorithm (mode_compute=8) if improving too slowly.