The add_offset CF property.
This property is only used when writing to a file on disk.
Examples
>>> f.add_offset = -4.0
>>> f.add_offset
-4.0
>>> del f.add_offset
>>> f.setprop('add_offset', 10.5)
>>> f.getprop('add_offset')
10.5
>>> f.delprop('add_offset')