fix urn computed for a slice, plus factor out lookup_members() from lookup_slice_members()
fix urn computed for a slice, plus factor out lookup_members() from lookup_slice_members()

tcmitchell/geni-ch,ahelsing/geni-ch,ahelsing/geni-ch,tcmitchell/geni-ch,MarshallBrinn/geni-ch,MarshallBrinn/geni-ch