Draft: Fixes the Zeeman term

Description

Fixes the Zeeman term

• The sign of the Zeeman term was not correct.
• The corresponding vector potential was correct, but due to the wrong phase of the Bloch states, we have to flip the sign of this vector potential too, in order to be consistent.

News snippet

Fixes the Zeeman term

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.