Define and implement a metric in `faro` that compares astrometry in the coadds between filters. (See Slack thread here.) Select a band (perhaps i-band) as a default reference (this should be configurable), and calculate statistics on matching unforced positions between g-band stars and i-band, r-band stars and i-band, etc. We should calculate both the net offset and scatter between the two catalogs, at the very least, and aggregate this per patch.

We should calculate all three of delta-RA, delta-Dec, and delta-r (2d separation).