Add output column to gAperture that contains the distance of the MCAT BG source used for mcatbgsub columns.

[scfleming]

Since the MCAT search radius expands, it's possible that the distance of the closest MCAT source to use for MCAT BGSUB can be quite different between visits, and it's important to know this if the background in the MCAT is being used from a source 0.1 deg. vs. 0.001 deg. for example.