An internal method to get a "default" covalent/element radius

        Args:
            site: (Site)

        Returns:
            Covalent radius of element on site, or Atomic radius if unavailable

def _get_default_radius(site):
        """
        An internal method to get a "default" covalent/element radius

        Args:
            site: (Site)

        Returns:
            Covalent radius of element on site, or Atomic radius if unavailable
        """
        try:
            return CovalentRadius.radius[site.specie.symbol]
        except:
            return site.specie.atomic_radius