'Update a  group.
:param context: the context of the caller.
:param group: the group object.
:param add_volumes: a list of volume objects to be added.
:param remove_volumes: a list of volume objects to be removed.
:returns: model_update, add_volumes_update, remove_volumes_update
ScaleIO does not handle volume grouping.
Cinder maintains volumes and CG relationship.'

def update_group(self, context, group, add_volumes=None, remove_volumes=None):

    if volume_utils.is_group_a_cg_snapshot_type(group):
        return (None, None, None)
    raise NotImplementedError()
