Fix problem with models where steadystate files change parameter values.

1) allow to compute derivatives starting from NUMERICAL derivatives of jacobian and steady state: this has a minor cost in accuracy and allow apply without errors identification and estimation with numerical derivatives;
2) added trap in dynare_estimation_init: if steadystate changes param values, automaticly shifts to numerical derivs of jacoban and steady state +  analytic derivatives of all the rest;
3) bug fixes for 2nd order derivatives w.r.t. model parameters;