Example on creating a single multi-robot factor graph using gtsam::LabeledSymbol

[Sandilya Garimella]

I am looking to do multi-robot pose graph optimization, where I have a single factor graph for all the robots and the landmarks that they detect. 

I looked at conversation "GTSAM on multi-robot SLAM" and noted that I need to add a betweenfactor between the robots because GTSAM cannot optimize disconnected graphs (singular system). I wanted to check if an example of using gtsam::LabeledSymbol can be provided and if there is an in-built way to keep track of previously observed landmarks.