Pipe AR marker pose to 2D nav goal

[matthew-reynolds]

• [ ] Publish static base_link->camera_centre TF
• [ ] Determine markers to follow (eg. maybe 0=follow, 1=stop, 2=squat, etc)
• [ ] Create state machine, where visible AR markers drive transitions. In "follow" state, enable nav stack, and create the nav goal by casting a ray ~2m from the marker 0 pose in the direction of the base link (So that we don't run into the target)