The flag_masks CF property.
Stored as a 1-d numpy array but may be set as array-like object.
Examples
>>> f.flag_masks = numpy.array([1, 2, 4], dtype='int8')
>>> f.flag_masks
array([1, 2, 4], dtype=int8)
>>> f.flag_masks = (1, 2, 4, 8)
>>> f.flag_masks
array([1, 2, 4, 8], dtype=int8)
>>> del f.flag_masks
>>> f.setprop('flag_masks', 1)
>>> f.getprop('flag_masks')
array([1])
>>> f.delprop('flag_masks')