Finding new subjects should check if an image has been updated

[mwalmsley]

find_new_catalog_images was originally written to

• check if a galaxy was in both catalogs (i.e. new?), and if so
• check if the fits files are different (i.e. updated?)

But I haven't yet downloaded all of DR2 (and I possibly don't want too). Therefore, checking if fits files are updated has been disabled.

I should either do the DR2 download and re-visit, or find another way to check if the galaxies have changed, or abandon.