Set the steady-state NOE spectrum type for pre-loaded peak intensities.
spectrum_type: The type of steady-state NOE spectrum, one of `ref' for the reference spectrum or `sat' for the saturated spectrum.
spectrum_id: The spectrum ID string.
The spectrum type can be one of the following:
Peak intensities should be loaded before this user function via the spectrum.read_intensities user function. The intensity values will then be associated with a spectrum ID string which can be used here.