Add option to generate shunting routes

[jeriox]

In the topologies imported from ORM we have a lot of Sperrsignale and also Hauptsignale which also can show some kind of shunting permission. It would be good to be able to generate routes to and from those signals as well. They could either be marked as shunting routes ("Rangierfahrstraßen") with a flag on the route or by using a separate subclass, as some route attributes like maximum_speed don't apply here.