I think this should become another todo, item, then: both `SpectralCube` and `Mask` should have a `with_wcs` option to cleanly create a copy where the only change is the WCS, and it should verify that said WCS are compatible with the data...  

I'm not sure how robust we can make that, actually, except to check that the axis order is appropriate, since WCS objects don't know about the data size.  Perhaps we should make that a private method since it has potential to break the underlying object if misused.