Use wheel normal force from joints instead of static parameter in WheelSlip plugin

[wittenator]

Desired behavior

In the current implementation of the WheelSlip plugin the wheel normal force is a static parameter. I think it would be beneficial to give an option to use the wheel normal force that the Gazebo physics engine reports. This would enable more flexible vehicle dynamics simulation and possibly allow the simulation of downforce via the LiftDrag plugin. If I am not mistaken in the current implementation additional forces do not propagate to the WheelSlip plugin and therefore don't influence driving behaviour.

Alternatives considered

None

Implementation suggestion

I think that adding an flag to the plugin that conditionally uses the force reported by the physics engine (as implemented in the force-torque sensor perhaps), would be a way to go.

[azeey]

@scpeters any thoughts on this?