[strands_movebase] Add launch parameters to change obstacle thresholds

This pull:

* Allows you to specify how high above the ground a point has to be to be counted as an obstacle (as opposed to floor, which is used for ray-tracing to clear obstacles). Via launch parameter `z_obstacle_threshold`.
* Same thing for stair detection, how low below the ground plane to be counted as stairs via the launch parameter `z_stair_threshold`.

This allows you to find a good balance between how sensitive a robot is to obstacles near the floor plane (low obstacles) and how robust the robot is. Our experience is that the navigation can be much more robust by setting e.g. `z_obstacle_threshold:=0.2` and `z_stair_threshold:=0.12`.