The test_axialExpansionChanger.py tests are too slow

[john-science]

The unit tests in test_axialExpansionChanger.py are the slowest in ARMI. For instance, these three tests alone take 30 seconds to run:

• TestInputHeightsConsideredHot::test_coldAssemblyExpansion - 15.84 seconds
• TestConservation::test_thermalExpansionContractionConservation_complex - 6.75 seconds
• TestManageCoreMesh::test_manageCoreMesh - 6.08 seconds

Could we reduce the size of the detailed axial expansion test reactor? That is clearly the cause of the slowness:

armi/armi/tests/detailedAxialExpansion/refSmallCoreGrid.yaml

Lines 4 to 20 in de4eeee

	- - SH
	- SH SH
	- SH OC SH
	SH OC OC SH
	OC MC OC SH
	OC MC MC OC SH
	MC MC MC OC SH
	MC MC SC OC SH
	MC PC MC MC OC SH
	IC IC MC MC OC
	IC IC MC MC SH
	IC LA IC MC OC
	IC IC SC MC SH
	LA IC MC OC
	IC IC IC MC SH
	IC IC MC OC
	AF IC IC PC SH

Do we really need a full 9-ring reactor to test that assemblies expand thermally, in the axial direction?