Deprecated method to collapse layers in the Overlay.

def collapse(self, function):
        "Deprecated method to collapse layers in the Overlay."
        if config.future_deprecations:
            self.param.warning('Overlay.collapse is deprecated, to'
                               'collapse multiple elements use a HoloMap.')

        elements = list(self)
        types = [type(el) for el in elements]
        values = [el.group for el in elements]
        if not len(set(types)) == 1 and len(set(values)) == 1:
            raise Exception("Overlay is not homogeneous in type or group "
                            "and cannot be collapsed.")
        else:
            return elements[0].clone(types[0].collapse_data([el.data for el in elements],
                                                            function, self.kdims))