Add option to compute/return Hessian in the C-API

[TyBalduf]

Describe the solution you'd like It would be useful if the Hessian could also be returned in C-API. While the Hessian is currently only computed numerically, its still much more convenient to have the 6N_atoms gradients computations handled in one call to xTB rather than needing to have an external loop and calling xTB 6N_atoms times.

If possible state how you can assist in providing data or code to to implement the feature Might be able to put together a pull request given enough time.