SpectralCoord can't handle pixel units

[jradavenport]

When I have 2 Spectrum1D objects, which are not wavelength calibrated and have pixels for the spectral unit, I cannot do e.g. addition or subtraction of my spectra. This is an issue for doing things like co-adding spectra, doing sky subtraction, etc.

Here is a MRE:

from specutils import Spectrum1D
import numpy as np
import astropy.units as u
t1 = Spectrum1D(spectral_axis=np.arange(100)*u.pixel, flux=np.random.normal(size=100)*u.adu)
t2 = Spectrum1D(spectral_axis=np.arange(100)*u.pixel, flux=np.random.normal(size=100)*u.adu)
t1-t2

which throws this error:
UnitTypeError: SpectralCoord instances require units equivalent to '(Unit("Hz"), Unit("m"), Unit("J"), Unit("1 / m"), Unit("km / s"))', so cannot set it to 'pix'.

I believe the workaround here is to do:

t1.subtract(t2, compare_wcs=None)

Could we expose this easier to users?