ENH: adding Gator derived dist column to TAP queries via a keyword

[bsipocz]

Gator provided some derived columns that are not returned in the default TAP response when querying. On the astroquery level the backend is transparent, so we may actually add an enhancement where we return at least some of the derived columns as part of an extended query. I would say we should only do this as an opt-in and only in the higher level API, e.g. query_region and not as part of query_tap.