Add diamagnetic and magnetization current densities

Description

The code now computes the diamagnetic and magnetization current densities, for the cases where there is a non-zero, non-uniform vector potential and when spin-polarization is considered, respectively. Unit tests against analytical expressions for complex LCAO of hydrogen orbitals were added.

Checklist

• Add test for magnetization current
• Rebase on main
• I have checked that my code follows the Octopus coding standards
• I have added tests for all the new features added in this request.

Closes #584 (closed)