Add segmentation sensor

[adlarkin]

Related to #135

Desired behavior

Add a segmentation sensor to Ignition. The goal is to create segmented image datasets that can be used in machine learning applications.

Alternatives considered

N/A (this is a new feature).

Implementation suggestion

We will need functionality added to the following repositories:

• https://github.com/ignitionrobotics/ign-gazebo
  • add a system plugin that allows users to specify labels for objects that they'd like to be segmented
• https://github.com/ignitionrobotics/ign-sensors
  • add the segmentation sensor that will publish segmented image data to a topic via ign-transport
• https://github.com/ignitionrobotics/ign-rendering
  • use rendering engines (like Ogre2) to actually generate a segmented image
• https://github.com/osrf/sdformat
  • add a segmentation sensor to the SDF specification

I don't believe that we will need any new message types added to https://github.com/ignitionrobotics/ign-msgs since we can make use of the image message type.

Once the sensor has been implemented, it would also be nice to add functionality for generating segmented image datasets. This will probably require manipulating a scene and saving generated image data to a ML-friendly format.

Additional context

A lot of work for implementing this sensor is already underway. Here are all of the related PRs that have been opened:

• Label Component & System gz-sim#853
• Segmentation Sensor #133
• Segmentation Camera gz-rendering#329
• Add segmentation and bounding box sensor types sdformat#592

[adlarkin]

@chapulina I believe we can close this issue, now that the Segmentation sensor has been merged for the Fortress release. Is there anything else we're missing? If not, feel free to close this.