Set the reference non-moving domain for the 2-domain frame order theories.
ref: The non-moving domain which will act as the frame of reference.
Prior to optimisation of the frame order model, the frame of reference non-moving domain must be specified. This is essential for determining which spins will be used in the analysis, which will be shifted to the average position, etc.
To set up the isotropic cone frame order model with `centre' domain being the frame of reference, type:
[numbers=none] relax> frame_order.ref_domain(ref='centre')