revisit maximum driveway length concept

[namdre]

At the moment driveways on fully bidirectional tracks are as long as the train route beyond the signal or at most --railsignal.max-block-length (default 20000m). This is done to avoid deadlocks with an oncoming train that makes use of the same tracks (as may happen when a train is assigned to the wrong track). This has the following problems:

• if a train should be assigned to the wrong track (rather than using an appropriate siding for passing), this manifests as departdelay and is less visible to the user (but the simulation result is likely not what the user intended)
• if all trains use the correct track and the maximum distance between two siding never exceeds max-block-length, then a useless warning is generated for long train routes

Proposed solution:

• only issue a warning if no siding was found within max-block-length
• optionally, abort driveway route construction after the first siding was found along the route (a side track that exits and re-enters the route)
  • maybe this should be the default behavior since it's more efficient
  • a warning should be issued if a siding track is used in both directions