Add missing parent quantity to electronic current

Description

In a recent update of the multisystem framework (!2760 (merged)), which added "parent" quantities, on which an on-demand quantity should depend. This prevents that quantities get 'ahead' of their parents. However, this MR did not actually add the parents for the current of the electron system, which is now done here.

As one can see in the plots below, the convergence of time-dependent data with the time-step is now much better, and as expected. In the old scheme (without declaring the parent) most time-dependent quantities lagged behind by one timestep.

Old scheme:

New scheme:

Due to these changes, we also needed to update the references of the test maxwell/12-tddft-current-to-maxwell.test.

News snippet

Add parent quantity of 'current' and 'dipole' in the electron system.

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.