Assigns site properties.

def assign_site_properties(self, slab, height=0.9):
        """
        Assigns site properties.
        """
        if 'surface_properties' in slab.site_properties.keys():
            return slab
        else:
            surf_sites = self.find_surface_sites_by_height(slab, height)
        surf_props = ['surface' if site in surf_sites
                      else 'subsurface' for site in slab.sites]
        return slab.copy(
            site_properties={'surface_properties': surf_props})