Do not compute the median of the coordinates of the solved fields
Remove --ra and --dec options
Add optparse.OptionGroup 'key_group'
Add --rak option
Add --deck option
Remove optparse.OptionGroup 'coords_group' — but keep --radius
Read α and δ of each image from its FITS header
Add DEBUG logging messages
Limit the search area to the coordinates read from the FITS headers

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