Buoy minor adjustments

[mabelzhang]

@seankrag I tagged you for a review since you are the most familiar with this code, but anyone else can feel free to review too.

I reviewed the buoy model addition after the fact and made some minor adjustments:

• Renamed the model for easier future maintenance (usually the artists name the models whatever they like, and we rename them to fit the context better)
• Credit the artist
• Style; remove dead code. I uncommented the PBR block, because style-wise, if some code is commented, then it should probably be removed. It generates a warning since Gazebo doesn't support PBR, but that's okay. Ignition does, and we can reuse the model in the future.

To test the model, add this block to perception.world.xacro:

    <include>
      <name>buoy_red</name>
      <pose>400 -550 1 0 0 0</pose>
      <uri>model://buoy_red</uri>
    </include>

Then run

$ colcon build --packages-select vorc_gazebo
$ roslaunch vorc_gazebo perception.launch

In the left panel, right-click on buoy_red > Move To. It should show up. According to the specs, it's supposed to be 1500 mm, so it will look about half the height of the boat. We can resize if we need to later.

[mabelzhang]

Thanks for checking! I only eyeballed. Merging!