FmuWrapper: Make AxleRatio available as AssignSpecial

Allows assignment of AxleRatio to FMI variables

doc/source/user_guide/sim_user_guide/components/vehicle.rst

@@ -223,6 +223,9 @@ The allowed special simulation values are as follows:
                                                                 This directory is not necessarily created by the FmuWrapper, depending on the setting of
                                                                 `CsvOutput` and `Logging` parameters (see `FmuWrapper basic configuration`_).
    MaxSteering                               Real               The `max_steering` property of an agent's front axle as defined in OpenSCENARIO (catalog).
+   AxleRatio                                 Real               The axle ratio of the vehicle model.
+                                                                Has to be defined in the properties of the Entity in the OpenSCENARIO catalog with the name
+                                                                ``AxleRatio`` and the value has to be a valid floating point expression.
    SteeringRatio                             Real               The steering ratio of the vehicle model.
                                                                 Has to be defined in the properties of the Entity in the OpenSCENARIO catalog with the name
                                                                 ``SteeringRatio`` and the value has to be a valid floating point expression.