Extracts reference and response DOF from an imat_fn.
[REF,RES] = ref_res(F)
REF_RES extracts reference and response DOF from imat_fn FRF. RES only includes the responses that are associated with ALL of the references.
F is an imat_fn containing FRF.
REF is an imat_ctrace reference DOF. RES is an imat_ctrace found associated with all reference DOF (REF).