changes to path_modification and ramp_planner
A Modifier class was added to path_modification. The Modifier class contains instances of all the modification operators. It is responsible for taking in a ModificationRequest and correctly applying a modification to the desired path(s).
A Population class and a RampTrajectory class were added to ramp_planner. RampTrajectory is needed so we can hold a ramp_msg::Trajectory and an associated fitness value in the same structure. The Population class will handle replacing trajectories after modifications have been performed. It will also call the evaluation function on the entire population.
Merge branch 'master' of https://github.com/sterlingm/ramp