code: public Vector3d half(double x, double y, double z) {
          return half(x, y, z, thisOrNew());
      }
docs: Compute the half vector between this and the vector <code>(x, y, z)</code>.
  
  @param x
  the x component of the other vector
  @param y
  the y component of the other vector
  @param z
  the z component of the other vector
  @return a vector holding the result