Ensure 2D spatial images can be loaded into spectral-cube

[e-koch]

Check that we can open 2D spatial images (w/ degenerate spectral and pol axes) and read that into a spectral_cube.Projection-like class (or elsewhere). This may only be a missing piece on the spectral-cube end but am noting it here as @keflavich adds more docs on 2D image handling in spectral-cube (radio-astro-tools/spectral-cube#834).