'Updates a group.
:param ctxt: the context of the caller.
:param group: the Group object of the group to be updated.
: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'

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

    if volume_utils.is_group_a_cg_snapshot_type(group):
        return self.update_consistencygroup(ctxt, group, add_volumes, remove_volumes)
    raise NotImplementedError()
