code: static Vector2d average(final Collection<Vector2d> vectors) {
          final Vector2d average = new Vector2d(0, 0);
          for (final Vector2d vector : vectors) {
              average.add(vector);
          }
          average.scale(1d / vectors.size());
          return average;
      }
docs: Average a collection of vectors.
  
  @param vectors one or more vectors
  @return average vector