Lighten Gaebo CPU load

[franreal]

Study ways of lighten Gazebo CPU load, as Daniel and Edu did modifying world physics. In the case world physics is the best solution, see if we should make a specific packege/folder for lighten worlds.

[jescap]

One solution is to have a world with camera and another without camera. There will be an example simulation for a given package, e.g., px4_bringup, and maybe specific simulations (worlds) for each particular project will go to their particular packages.