class decorator to Register mf make

def _register_make(cls):
    '''class decorator to Register mf make'''
    assert cls.nxm_headers is not None
    assert cls.nxm_headers is not []
    for nxm_header in cls.nxm_headers:
        assert nxm_header not in _MF_FIELDS
        _MF_FIELDS[nxm_header] = cls.make
    return cls