Trajectory reference state

[ChamzasKonstantinos]

Currently the majority of the Trajectory functions require a reference state. This happened due to the MoveIt not including the reference state in the msg 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 which would not break the constructor API but we would still need the reference state for OMPL to MoveIt trajectory

Opening for discussion/ideas