code: public static void changeSign( DMatrix3 a )
      {
          a.a1 = -a.a1;
          a.a2 = -a.a2;
          a.a3 = -a.a3;
      }
docs: <p>
  Changes the sign of every element in the vector.<br>
  <br>
  a<sub>i</sub> = -a<sub>i</sub>
  </p>
  
  @param a A vector. Modified.