Implement derived trajpts

Not all methods are fully implemented, but it should be a start. Should also mostly address issue #4.
At this point, the only use case I see for getNominalCartPose() method is finding 6DOF distance between adjacent trajectory points. This would be useful for, say, deciding how the path needed to be discretized, or setting a rough velocity.  This is not the call to make in order to do arm IK.