Trajectory reference state

Currently the majority of the [Trajectory](https://kavrakilab.github.io/robowflex/classrobowflex_1_1Trajectory.html) functions require a reference state. This happened due to the MoveIt not including the reference state in the [msg](http://docs.ros.org/en/melodic/api/moveit_msgs/html/msg/RobotTrajectory.html) representation.
One idea is that the reference state could become a member variable to reduce the complexity of the signature functions.
We could read the reference state in the constructor, and add getters/setter functions to modify it.
Another idea is to use the DisplayTrajectoryMsg [msg](http://docs.ros.org/en/melodic/api/moveit_msgs/html/msg/DisplayTrajectory.html) which would not break the constructor API but we would still need the reference state for OMPL to MoveIt trajectory

Opening for discussion/ideas