Removed useless routines for gradient evaluation (called by csminwell).
Changed logic in numgrad2.

Do not compute the left derivative if the evaluation of the objective function at x+h returns a non zero cost_flag (ie a problem with BK, steady state, ...).
Cosmetic and efficiency changes.
Changed logic in numgrad3 and numgrad5 (same as for numgrad2) + Cosmetic and efficiency changes.
Added new routines for computing gradients (options 13 and 15).