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
By the way, I just realized, ``CAR`` isn't a guarantee of no distortions, there could be distortion coefficients. Maybe it is excessive to have two functions - how about just providing the ``reference`` one and then it's up to whoever builds tools around this to emit warnings if they want?