removes degenerate and ghost magnon modes from spectrum
spec = sw_omegasum(spec, 'Option1', Value1, ...)
It removes the degenerate modes from the dispersion stored in
spectra.omega and sorts omega according to the energy.
The degenerate dispersion energies are substituted with NaN values. Be
carefull, after this function sw_egrid() won't work properly on spectra.
Options:
tol Tolerance, within two energies are considered equal. Default
value is 1e-5.
zeroint The minimum intensity value, below the mode is dropped. Default
value is zero (no modes are dropped due to weak intensity).
See also SW.SPINWAVE, SW_EGRID.