Added falsePositives to mag check

The mag check script computes the magnitudes of the detected artificial sources in the image.
Now it also computes the magnitudes for other things in the image that were not added to the images, sort-of
the false positives.
Corrected the use of ZP for flux computing.

The update_astrometry script should use the ZP in the astrometric header when computing magnitudes, not the original image header.
Now the script does that.
Merge remote-tracking branch 'origin/master'