changed Galapsgo sky estimation. This can now deal with entirely masked images, e.g. when bands have different footprints.

Additionally, added some more sky output, e.g. flags and everything are now stored in the output catalogues.

bugfix:
B/D readout does not overwrite the SS sky values anymore, but gets it's own columns. A lot of information is in the catalogues twice now, though.
Merge branch 'master' of github.com:MegaMorph/galapagos