code: public void swapRows(int r1, int r2, Object tmp)
      {
          int col = columns();
          System.arraycopy(array, col * r1, tmp, 0, col);
          System.arraycopy(array, col * r2, array, col * r1, col);
          System.arraycopy(tmp, 0, array, col * r2, col);
      }
docs: Swaps row r1 and r2 possibly using tmp
  @param r1
  @param r2
  @param tmp double [columns]