Add vehicle access equipments

[Robbendebiene]

Resolves #873

In general the proposal follows the idea of place equipments like LiftEquipment or RampEquipment which all derive from an abstract AccessEquipment.

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

• Dropped MinimumEquipmentLength and MaximumEquipmentLength to avoid choice element in favour of IsAdjustableLength. Also the minimum length for sliding steps and ramps would usually be close to 0 which isn't very valuable.
• Added HoistOperatingRadius to HoistVehicleEquipment for feature parity with AccessVehicleEquipment
• Added NumberOfTreads to RetractableStepVehicleEquipment to clarify some scenarios (see aa7c6e3674ec9f9b622cc056f9ffe02e278709c0)
• Modified AccessVehicleEquipment & WheelchairVehicleEquipment to reuse PassengerEquipmentGroup in order to avoid code duplication (see 563fdabf861ca3061a6207dfb067fbf17ae3db9f)
• Dropped KneelingBoardingHeight in favour of a more flexible modelling way
• Added real ramp length to reliably calculate the ramps slope because the external length can differ from the ramps length (example)
• Used "External" instead of "Equipment" to describe outer equipment dimensions