HW31
  Python version
  Solve the PDE with 6, 11, and 21 nodes.
  Compute and print the L2 and H1 errors.

  Nodes       L2              H1

   6        0.017362        0.275139
  11        0.004387        0.138818
  21        0.001100        0.069566
  41        0.000275        0.034803
  81        0.000069        0.017404