improve param read in velocity polygon

[papertowel123]

reason for change: added the ability to set certain parameters for each sub-polygons (for limit, slowdown and approach type of velocity polygons)

so now, for each sub-polygon we can specify the slowdown_ration parameter:

    VelocityPolygonStop:
      type: "velocity_polygon"
      action_type: "slowdown"
      min_points: 6
      visualize: True
      enabled: True
      polygon_pub_topic: "velocity_polygon_stop"
      velocity_polygons: ["rotation", "translation_forward", "translation_backward", "stopped"]
      holonomic: false
      rotation:
        points: "[[0.3, 0.3], [0.3, -0.3], [-0.3, -0.3], [-0.3, 0.3]]"
        linear_min: 0.0
        linear_max: 0.05
        theta_min: -1.0
        theta_max: 1.0
        slowdown_ration: 0.4
      translation_forward:
        points: "[[0.35, 0.3], [0.35, -0.3], [-0.2, -0.3], [-0.2, 0.3]]"
        linear_min: 0.0
        linear_max: 1.0
        theta_min: -1.0
        theta_max: 1.0
        slowdown_ration: 0.2
      translation_backward:
        points: "[[0.2, 0.3], [0.2, -0.3], [-0.35, -0.3], [-0.35, 0.3]]"
        linear_min: -1.0
        linear_max: 0.0
        theta_min: -1.0
        theta_max: 1.0
        slowdown_ration: 0.1

[jplapp]

I think this approach is now incompatible to the upstream approach,so we wont be able to push this. We should make it so that these velocities are optional: If they are not defined for the polygon, the "main" ones should be used instead. But let's do that as a follow-up step