Support for magnitude limits in Source files

Currently the Source file format supports storing magnitude bands as a FITS binary table with 3 columns:

• filter band
• magnitude
• magnitude error

It would be useful to also be able to assign magnitude limits, i.e. when an object is not detected in a particular band.

Perhaps two extra columns could be added:

• upper magnitude limit
• upper magnitude limit error

Since the default values should be NaN if not provided, I imagine it should be possible to retain some backward compatibility for reading older Source file versions.