trajectory_generator and path_modification changed to services
The trajectory_generator and path_modification packages are changed to service servers, rather than working with msg topics.
We were essentially reimplementing srv calls before with the publish+subscribe classes.
The code is cleaner and easier to understand now that they are changed to service servers.
ramp_planner kept the handlers for trajectory and modification requests, but the handlers now act as service clients, rather than working with msg topics.