code: public static Trajectory combineTrajectory(Trajectory a, Trajectory b){
  		if(a.getDimension()!=b.getDimension()){
  			throw new IllegalArgumentException("Combination not possible: The trajectorys does not have the same dimension");
  		}
  		if(a.size()!=b.size()){
  			throw new IllegalArgumentException("Combination not possible: The trajectorys does not "
  					+ "have the same number of steps a="+a.size() + " b="+b.size());
  		}
  		Trajectory c = new Trajectory(a.getDimension());
  		
  		for(int i = 0 ; i < a.size(); i++){
  			Point3d pos = new Point3d(a.get(i).x+b.get(i).x, 
  					a.get(i).y+b.get(i).y, 
  					a.get(i).z+b.get(i).z);
  			c.add(pos);
  		}
  		
  		return c;
  	}
docs: Combines two trajectories by adding the corresponding positions. The trajectories have to have the same length.
  @param a The first trajectory
  @param b The second trajectory
  @return The combined trajectory