Deprecated method to apply new group to items.

        Equivalent functionality possible using:

            ViewableTree(tree.relabel(group='Group').values())

def regroup(self, group):
        """Deprecated method to apply new group to items.

        Equivalent functionality possible using:

            ViewableTree(tree.relabel(group='Group').values())
        """
        if util.config.future_deprecations:
            self.param.warning('%s.regroup is deprecated, use relabel '
                               'method with a group argument instead.'
                               % type(self).__name__)
        new_items = [el.relabel(group=group) for el in self.data.values()]
        return reduce(lambda x,y: x+y, new_items)