Publish GPS data from the game

[tckarenchiang]

There are a few Gazebo plugins which do something similar:

• hector_gazebo_plugins#GazeboRosGps (also mentioned in Gazebo simulation GPS fix)
• Gazebo itself should now (with recent versions) have its own version of a "GPS Sensor": ethz-asl/rotors_simulator#325. Haven't really been able to find much about it though. That is separate from the Hector plugin mentioned above.

These could be interesting to see how they approach this (ie: map in-game/simulation coordinates to GPS coordinates).

Adding noise to the GPS output would also be interesting (to avoid "perfect" data): https://dev.px4.io/v1.9.0/en/simulation/gazebo.html#simulating-gps-noise