Fixed lib.topology.Element to have a set of allowed address types and guard against accidental construction with no address type
Fixed lib.topology.Element to have a set of allowed address types and guard against accidental construction with no address type

dmpiergiacomo/scion,klausman/scion,dmpiergiacomo/scion,netsec-ethz/scion,klausman/scion,FR4NK-W/osourced-scion,FR4NK-W/osourced-scion,netsec-ethz/scion,dmpiergiacomo/scion,dmpiergiacomo/scion,dmpiergiacomo/scion,netsec-ethz/scion,klausman/scion,netsec-ethz/scion,Oncilla/scion,FR4NK-W/osourced-scion,klausman/scion,Oncilla/scion,FR4NK-W/osourced-scion,FR4NK-W/osourced-scion,Oncilla/scion,Oncilla/scion,dmpiergiacomo/scion,klausman/scion