Examples all compile

o Deleted Hazeltine 4-field model, as incomplete

o Reset the benchmark data for test-laplace. I think the output is correct,
  but the new boundary conditions seem to have changed the results considerably
  in some cases. -> All test suite cases should now pass

o Small fixes so that other examples compile. Haven't checked that they all run though!

o Added MMS diffusion and advection tests to test suite, since they
  run very quickly.

o In addition the diffusion2, advection and H-W MMS tests still run and produce expected output