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
Yes - I'm suggesting have one function, the "reference celestial scale", that will never raise a warning, and this function, which wraps that one, that will raise a warning if.  Reasonable, or excessive?