Limit the search area to the coordinates read from the FITS headers

The idea of, for each image, using as its approximate coordinates the
median of the right ascension and declination of the fields that have
already been solved imposes a severe limitation: it forces that all
the FITS images for which an astrometric solution is being obtained
belong to the same field. We can do better than that — although the
FITS standard does not define them, we have yet to come across an
image without the RA and DEC keywords, widely used within the
astronomical community and that we can use to limit the search area.

This merges branch 'read-ra-and-dec-from-FITS-header'