Thermal Expansion should be a Plugin / Interface

[john-science]

The thermal expansion logic in ARMI seems to hyper-specific to Sodium Fast reactor designs. That is counter to the ARMI design goals of being a tool useful to the general public.

Of course, the Thermal Expansion logic in ARMI is heavily used by some of our users/developers, we don't want to just lose that support.

The solution, put forward by @keckler, is that we should encapsulate our Thermal Expansion into a Plugin and/or Interface.

This refactor seems useful and thus valuable.

[john-science]

Also, we had a whole discussion on this before: https://github.com/terrapower/armi/discussions/766