The flag_values CF property.
Stored as a 1-d numpy array but may be set as any array-like object.
Examples
>>> f.flag_values = ['a', 'b', 'c']
>>> f.flag_values
array(['a', 'b', 'c'], dtype='|S1')
>>> f.flag_values = numpy.arange(4)
>>> f.flag_values
array([1, 2, 3, 4])
>>> del f.flag_values
>>> f.setprop('flag_values', 1)
>>> f.getprop('flag_values')
array([1])
>>> f.delprop('flag_values')