New models

[bsb808]

This PR adds three new object models (torpedo, flight-data-recorder, sonobuoy and three unexploded ordnance) along with a demonstration world.

Reviewer should be able to run the command

roslaunch dave_demo_launch dave_models.launch 

and view a minimal world with the three now models on the seafloor:

Please check that the link coordinate frames make sense and that the collision meshes are appropriate. I find it easiest to use the Gazebo gui to view this attributes.

[mabelzhang]

Someone else who knows what's "correct" should review this. I just took a brief look to steal the SDF file to test other models.

Coordinate frames look right - assuming all of them being Z up X forward is what we want.

Inertia boxes look off. Do we care about that? If we are planning to use them for manipulation, that might not be negligible.

Side note, I see the light/dark shadow upstream bug now... Guess I have the dark version.

Also saw another upstream bug that if I enable skeleton view, then disable it, then enable inertia view, all the magenta boxes are opaque as opposed to translucent. Probably not an urgent thing to fix. FYI @WilliamLewww

[bsb808]

@mabelzhang Good catch on the inertia values. I improved them. Still coarse estimates, but I think they should be sufficient for dave.

[mabelzhang]

Looks good. Did you intentionally skip torpedo_mk46 for the inertials? Wanted to double-check before approving.

[mabelzhang]

Oh never mind, that one was already correct.

Thanks for adding them!