code: public static void diag( DMatrix3x3 input , DMatrix3 out ) {
          out.a1 = input.a11;
          out.a2 = input.a22;
          out.a3 = input.a33;
      }
docs: <p>
  Extracts all diagonal elements from 'input' and places them inside the 'out' vector. Elements
  are in sequential order.
  </p>
  
  
  @param input Matrix.  Not modified.
  @param out Vector containing diagonal elements.  Modified.