Private convenience method for get_nn_info,
        gives original site index from ProvidedPeriodicSite.

def _get_original_site(structure, site):
        """Private convenience method for get_nn_info,
        gives original site index from ProvidedPeriodicSite."""
        for i, s in enumerate(structure):
            if site.is_periodic_image(s):
                return i
        raise Exception('Site not found!')