Add additional vehicle entrance dimensions

[Robbendebiene]

Resolves #900

This slightly deviates from the original proposal in the following ways:

• Add StepHeight to AccessVehicleEquipment (required by German handbook of accessible travel chains - see code 3111)
• Add EdgeToTrackCenterDistance to VehicleType and AccessVehicleEquipment to allow modelling/calculating the horizontal gap withot the necessity of having a DeckPlan.