getWaypointTime returns the input time for each waypoint [s]
 
    This method returns the time for each waypoint that served
    as input to calculate this trajectory. 
 
    If the time vector has been specified manually, the result 
    is equivalent to:
 
        offset = time(1);
        result = ((time - offset) * speedFactor) + offset
 
    This is useful for, e.g., plotting the waypoints on top of
    a whole trajectory.