ENH: add search offset column to Heasarc.query_region

[zoghbi-a]

The returned table does not have a an offset or distance from the requested RA and DEC.
The end user can work it out using SkyCoord, but it would nice to have it as an extra column

[zoghbi-a]

I should add that this makes sense only querying in circular region around some position (spatial='cone'), but it is likely the most common case anyway.

[zoghbi-a]

I forgot that this is already implemented. query_region already has add_offset option. It just needs to be documented.