LocalADOLCStiffness: use "hessian2" instead of "hess_vec"

With this commit, we do two things:

• replace "hess_vec" with a simpler call of "hessian" which calls internally "hess_vec".

• use "hessian2" by default instead of "hessian". "hessian2" calls internally "hess_mat" which is faster than "hessian/hess_vec". If the user encounters probems with the new "hessian2" driver, a boolean "useHessian2" can be set to false in the constructor.