More WCS convenience functions: pixel_scale, celestial

Some more WCS convenience attributes:

`wcs.pixel_scale` returns the pixel scale ("plate scale") in degrees in the celestial dimensions *if* the pixels are symmetric (otherwise, it raises an exception)
`wcs.celestial` returns a copy of the celestial component of the WCS

Question: Should `wcs.pixel_scale` return a `Quantity`?

cc @mdboom
Maybe 'square' instead of 'symmetric' (since it's not really symmetry)