Allow users to query a list of coordinates for SDSS query

Due to restrictions by the SDSS server it can be useful to query a list
of coordinates in a single query. The result can then be matched with
the original list by using:

```
dr = coords.Angle(radius).to('degree').value
obj_coords = coords.SkyCoord(result['ra'], result['dec'], frame='icrs', unit='deg')
matches, d2d, d3d = obj_coords.match_to_catalog_sky(coordinates)
result = result.group_by(matches)
```

where the group keys are the indices to the coordinates sent to the
query.

A future modification of astroquery can do the above procedure
automatically.
The formatting is a bit awkward here since `if (` has 4 characters; my approach to distinguishing the post-`:` line from the line before is to add an extra set of parentheses.  This is really minor, and I'm open to suggestions, including just leaving it alone.