RoutingTableVisualizerBase can be genericised for Ipv6 and NextHopRoutingTable

`RoutingTableVisualizerBase::addRouteVisualizations(IIpv4RoutingTable)` => `addRouteVisualizations(IRoutingTable)`

It already only uses `IRoutingTable` interfaces. 

`RoutingTableVisualizerBase::getDestinations()` get Destinations can also be genericised to return `vector<L3Address>`