Optimizes the local potential and the magnetic term for spinors.

Description

Optimizes the local potential and the magnetic term for spinors.

Fixes a bug for the combination of spinors and imaginary potential.

News snippet

Optimization for spinors and for calculation with a non-uniform vector potential like in presence of a uniform B field.

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.