code: public static void swapCol(Matrix A, int j, int k)
      {
          swapCol(A, j, k, 0, A.rows());
      }
docs: Swaps the columns <tt>j</tt> and <tt>k</tt> in the given matrix.
  @param A the matrix to perform he update on
  @param j the first column to swap
  @param k the second column to swap