Use Zs3 as measure for route speed

[jeriox]

Consider the following scenario:

We know that the main tracks are rated for 120 km/h, but we don't know the maximum speed for the tracks in between. On the other hand we know that the signals are equipped with a Zs3 that can show a speed limit of 60 km/h. This allows the conclusion that the connecting tracks are rated for 60 km/h. We could use that information to improve the data quality on the routes. However, this is also a bit tricky to implement. The current idea would be to set the route speed if there is exactly one Zs3 symbol on the start signal and the route contains at least one edge without a maxspeed set. This could also lead to routes erroneously being set to a lower speed than in reality (e.g. if the main track speed was not set in the example above, this would lead to all routes being set to 60 km/h)