Add stereo camera to unity simulation (depth-map)

[Alb33rt]

The ZED camera in real-life provides built in depth-map and visual slam capabilities. This means we should be able to use this in conjunction with our main ROS system. However, this means that we will probably need to have the same feature-set implemented in the simulation. This is similar to how Software-in-the-Loop testing works. We replace the ZED cam with a virtual black-box implementation of Visual SLAM and test the rest of our code.