Set the reference domain for the `2-domain' N-state model.
ref: The domain which will act as the frame of reference. This is only valid for the `2-domain' N-state model.
Prior to optimisation of the `2-domain' N-state model, which of the two domains will act as the frame of reference must be specified. The N-states will be rotations of the other domain, so to switch the frame of reference to the other domain simply transpose the rotation matrices.
To set up a 5-state model with `C' domain being the frame of reference, type:
[numbers=none] relax> n_state_model.ref_domain(ref='C')