The flag_values CF attribute.
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, dtype='int8')
>>> f.flag_values
array([1, 2, 3, 4], dtype=int8)
>>> f.flag_values = 1
>>> f.flag_values
array([1])