Add energy conservation test

[hsaunders1904]

Summary

Adds a test to check that energy is conserved in a simple conjugate heat transfer problem. See the README in the test's directory for a description of the problem.

Checklist

• Tests have been written for the new/changed behaviour.
• Documentation/examples have been added/updated for the new changes.

[hsaunders1904]

This test currently fails, I'm not completely sure what is causing the difference in the energies yet.

Here's the test failure:

======================================================================
FAIL: test_moose_and_foam_energy_deltas_are_equal (test.TestEnergyConservedInSimpleConductionExample)
----------------------------------------------------------------------
Traceback (most recent call last):
  File ".../hippo/test/tests/multiapps/energy_conserved_in_simple_conduction_example/test.py", line 36, in test_moose_and_foam_energy_deltas_are_equal
    self.assertAlmostEqual(foam_de, -moose_de)
AssertionError: 238.70727749797516 != 222524.174114801 within 7 places (222285.466837303 difference)
----------------------------------------------------------------------
Ran 1 test in 0.500s

FAILED (failures=1)

There's some debugging code in the Python test script that plots the change in energy over time for each domain, the plots look like this:

The shapes of the curves look promisingly similar, but the scales are pretty off.