Handle superroutes relations

[Phyks]

Version 1 of https://www.openstreetmap.org/relation/9715471/history for instance

[Phyks]

Superroutes are not supported by osm2pgsql and the members routes are already drawn: osm2pgsql-dev/osm2pgsql#101. Not sure what should be done here, but not much can be done without osm2pgsql support. :/

The only useful use case I find so far is EuroVelo which is represented as a relation of national relations (see https://www.openstreetmap.org/relation/4774799). This is not an issue for CyclOSM since these road are already fully represented by the underlying roads.

Therefore I'm closing this one for the moment.