Accept multiple input FITS images and the output LEMONdB
Remove --output option
Fix error message displayed when the annuli XML file does not exist

The error message incorrectly shows the path to the offsets XML file,
but the error is caused if the --annuli option is used but the annuli
XML file does not exist.
Add --filterk option
Do not load FITS images from the offsets XML file
Fix wrong message — we are doing astrometry, not photometry
Map each photometric filter to its respective FITS images
Fix typographical error — it's "does not include", not 'included'
Display a progress bar as the filter of each FITS file is read
List how many FITS files belong to each photometric filter
Add --datek option
Add --timek option
Map each FITS image to its date of observation (UTC)
Add InputFITSFiles class
Add InputFITSFiles.__iter__()
Add InputFITSFiles.__len__()
Add InputFITSFiles.remove()
Add class docstring
Merge branch 'add-InputFITSFiles' into update-photometry
Use InputFITSFiles to store the images received as input