Specify that one tensor is a reduction of another.
full_tensor: The full alignment tensor.
red_tensor: The reduced alignment tensor.
Prior to optimisation of the N-state model and Frame Order theories using alignment tensors, which tensor is a reduction of which other tensor must be specified through this user function.
To state that the alignment tensor loaded as `chi3 C-dom' is a reduction of `chi3 N-dom', type:
[numbers=none] relax> align_tensor.reduction(full_tensor='chi3 N-dom', red_tensor='chi3 C-dom')