Fixed instability in runLQR in UnderwaterAcrobot example

The default geometry in examples\UnderwaterAcrobot was poor, largely because I assumed a 2D scenario on the fluids side. The example has been corrected to now represent two beams of circular cross-section, rather than the original flat ellipses. The plant, visualizer, and URDF files have been updated with this new geometry.

The system is also now representative of something one would actually build. This change allows the system to be controlled at a more reasonable control cost in UnderwaterAcrobotPlant.balanceLQR(), which I think caused the original instability issue.