SIMULATION: simulate velodyne lidar in gazebo

[kev-the-dev]

There is a velodyne_simulator program that we copied into the repo. With #355, this is currently not used in gazebo.

We should run the GPU-accelerated version of the velodyne simulator. However, I believe this will cause beams to reflect of the water. This can be modified to filter out points at the water level (besides some random noise?) to provide a more realistic simulation.

Consider adding an option to disable the velodyne simulation at runtime, as it will significantly slow the simulator.

[kev-the-dev]

There is an issue on vmrc for this. One solutions is using OGRE viewports.

Here's some notes:
Visuals (CollisionVisuals is likely the one for ocean) have a set visibility flag function where you can set a mask for what things see it.

GpuRaySensor has a OgreViewport function which gives you the OgreViewport which has a setVisibilityMask function.

Setting the ray sensor to a mask which does not contain the bit set in the collision should make it disappear from the ray. This could be implemented as a sensor plugin which accepts a list of models/collisions to hide from the sensor.