Added functionality for the trajectory_generator to publish a Trajectory whenever it receives a TrajectoryRequest.
The Planner class can build a Trajectory msg based on a path it maintains
trajectory_generator subscribes to /traj_requests and publishes to /trajectories
ramp_planner subscribes to /trajectories and publishes to /traj_requests
TODO: ramp_planner must be able to automatically determine what TrajectoryRequests incoming trajectories are associated with.