Document buoyancy action / VBS

[chapulina]

Part of #48

• Document buoyancyAction and buoyancyPosition. Looking at the code history, it looks like at some point cc (cubic centimeter) was used as a unit, but AFAICT, both our plugins and the controller are consistently using cubic meters, so I documented this.
• Made the LRAUV_example_buoyancy more flexible so we can test different volumes.
• Checked that test_buoyancy_action has correct expectations
• Updated the TODO in the buoyancy_engine link, becuase I believe we have a different issue now.

It would be good to have some eyes on this PR by people who have been involved in the original implementation (@mabelzhang / @arjo129 ), just to make sure I didn't miss anything related to units and the inertial TODO.

[mabelzhang]

Yeah we changed cc to m^3 to be consistent with Ignition units. Initially we used cc because that's how MBARI refers to the volume, both in speech and in code.

I looked over the code, but Arjo should have a look since he did the implementation.

[arjo129]

Thanks! Tested the changes to the example and it works as expected.