Add method for computing the hessian of a scalar-valued function

[marcelluethi]

This PR proposes two changes:

1. To add a hessian method, which computes the hessian matrix for a (tuple) of tensors.
2. To add a ravel method to TensorTree, which can be used to flatten/unflatten a TensorTree to a Tensor1. This is useful in its own right, but the motivation here is as a support function for Hessian and Jacobian methods, which can only work with (tuple) of tensors, but not case classes.