Add  Skycoord.to_pixels_wcs

This is a fairly trivial add on to #2885 that adds a convinence method to go from SkyCoord to pixel coordinates.

Not that there is *not* an inverse method, because it would make most sense as a classmethod, and `astropy.wcs.utils.pixel_to_skycoord` only uses the `SkyCoord` object explicitly.  It wouldn't be too hard to change that, though... What do you think, @astrofrog?
Yeah, this should be generally avoided unless it's absolutely necessary.