Use Dynare (trustregion) nonlinear solver for computing the steady state.

 - Added trustregion solver (close to minpack, the main difference
   with minpack implementation is that the Jacobian is computed on
   each iteration).
 - Added unit test for trust region solver (rewrote minpack
   testsuite). All the examples are working.
 - Added unit test for the steady state numerical solver.
 - Updated the preprocessor.