code: SparseVector centroid_vector()
      {
          if (documents_.size() > 0 && composite_.size() == 0)
              set_composite_vector();
          centroid_ = (SparseVector) composite_vector().clone();
          centroid_.normalize();
          return centroid_;
      }
docs: Get the pointer of a centroid vector.
  
  @return the pointer of a centroid vector