[Garden] Fix issue #563 and inertias

[caguero]

Fix issue #563.

The problem was in the wamv_pinger.xacro. We were creating a visual with an offset equals to the position of the pinger in world coordinates. This is wrong as the visual (if any) should represent the acoustic sensor and it should be inside the wam-v.

I removed the visual and its collision as it's not very useful.

Bonus: I added some missing inertias, otherwise the default inertia values were incorrect. The inertias look good now:

How to test it?

• Recheck the instructions described in [Garden] Port AcousticPinger plugin #531 to confirm that the pinger is still working correctly.
• Click on the WAM-V. Verify that the bounding box is now correct.
• Right click and select View->Inertia to confirm that the inertias are reasonable. Note that the inertia visualization is very slow the first time that you request it. This is a Gazebo issue.

[M1chaelM]

• The bounding box looks good.
• Inertia looks good (and makes a big difference in the behavior of the wam-v!).
• The pinger appears to still be working as expected