[DynareDev] Estimated params

Hi Stéphane (and everybody else),

I just committed the change to allow arbitrary expressions in estimated_params block (and also in estimated_params_init and estimated_params_bounds).

Previously only constants were allowed there.

This involved more modifications than I initially thought, so please test it a little bit to make sure that I didn't break anything (I tested it on fs2000.mod only).

Also note that by the way I fixed a bug when estimated_params_init and estimated_params_bounds were used in conjunction with correlations of endogenous variables; I also applied the fix to the 4.0 branch.

Best

Sébastien