Add functionality that generates the symbolic Hessian of the Lagrangrian

[moorepants]

We currently rely on Ipopt's internal algorithm for numerically computing the Hessian of the Lagrangian. We could generate this symbolically as we do for the Jacobian of the constraints and gradient of the objective and likely improve performance for Ipopt.

cyipopt docstring:

[moorepants]

Possible the cyipopt docs do not quite have the right math equation, from Ipopt:

[moorepants]

From Ipopt docs: "We also need to determine the Hessian of the Lagrangian. (If a quasi-Newton option is chosen to approximate the second derivatives, this is not required. However, if second derivatives can be computed, it is often worthwhile to let Ipopt use them, since the algorithm is then usually more robust and converges faster."