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?
No import cycle problem here, but my attitude here is that it makes the code cleaner because `skycoord_to_pixel` is pretty unlikely to be used in any other method.  So it makes things less cluttered to have it at function-level instead of module level.