Introduce batch function for gradient

Description

This batch function computes the gradient for a batch and also takes care of correcting for non-orthogonal cells (not yet on GPUs).

It is now used in the batch version of the current.

This is needed for the computation of the curl, needed for the Maxwell Hamiltonian.

News snippet

Batch version of gradient

Checklist

• I have checked that my code follows the Octopus coding standards
• I have added tests for all the new features added in this request.