This variable selects which magnetic constrain expression is added to the Hamiltonian.
The AtomsMagnetDirection block is used for determining the constrained direction.
This is ignored for independent particles, as in this case, there is nothing that
determines the magnetic direction.
Options:
constrain_none:
No constrain is added to the Hamiltonian.
constrain_dir:
We are adding a constrain for the direction and sign of the magnetic moments only,
see Ma and Dudarev, PRB 91, 054420 (2015).
constrain_full:
We are adding a constrain for the direction and norm of the magnetic moments only,
see Ma and Dudarev, PRB 91, 054420 (2015).