initialize ranges with max_range rather than zero

[fmessmer]

situation before:

• cob_sick_s300 driver publishes some ranges as 29.959999084472656 - most likely due to reflections e.g. from railings or glass fronts
• these data points are removed during the projection in cob_scan_unifer - because they are >max_range (max_range=29.5)
• the unified_scan.ranges are initialized with 0.0
• for those data points removed during projection, scan_unified will have a range of 0.0 which later gets removed during laser_zone_filter resulting in "holes" in the scan
• along the rays within those "holes" the costmap cannot clear the obstacle layer

situation now:

• now scan_unifier behaves as the driver: fake data points are set to max_range

this significantly reduces "trails" in the costmap for the scan unified obstacle layer

I already verified the behavior on cob4-22 in 25h köln....and it looks much better

contributes to https://github.com/mojin-robotics/cob4/issues/888#issuecomment-439805060

[HannesBachter]

Please add a comment (in the code) on why you changed the default distance to max_range in order to avoid confusion in a future investigation