Modifier class in ramp_planner
Created the Modifier class in ramp_planner. It will handle all things modification related.
The ModificationRequestHandler member in planner is replaced by a Modifier member.
The Modifier class will hold a ModificationRequestHandler so it can communicate with path_modification, rather than ramp_planner doing that.
TODO: Need a way to efficienly update the Modifier with updates to the planner's population. We don't want to have to completely reassign the vector<Path> each time we want to modify.