carla获取rpm

@see https://github.com/carla-simulator/carla/issues/2989

        c = vehicle.get_control()
        p = vehicle.get_physics_control()

        # TODO: proper simulation
        # It is still unclear to me how the gears are modeled in CARLA:
        #   -1 reverse
        #   0  neutral
        # But how does this map to the gear ratios?
        engine_rpm = p.max_rpm * c.throttle
        if c.gear > 0:
            gear = p.forward_gears[c.gear]
            engine_rpm *= gear.ratio