Refactor Axial Expansion

[john-science]

What is the change?

This PR refactors of the axial expansion functionality in ARMI. A new directory is structure is created to enable better traceability with testing and general maintainability. Some imports are added to the new __init__.py file to perfectly preserve the ARMI API, so that this change does not require any downstream changes.

Original

armi/reactor/converters/
|-- axialExpansionChanger.py

New

armi/reactor/converters/axialExpansionChanger
|-- __init__.py
|-- assemblyAxialLinkage.py
|-- axialExpansionChanger.py
|-- expansionData.py

Why is the change being made?

The axialExpansionChanger.py file was pretty long, and there is potentially a lot of work coming up on this file. So we are trying to make the future work cleaner.

I just wanted to do this now, as I see a lot of axial expansion work happening soon. Let's just get the necessary cleanup work out of the way.

---

Checklist

• This PR has only one purpose or idea.
• Tests have been added/updated to verify any new/changed code.

• The code style follows good practices.
• The commit message(s) follow good practices.

• The release notes have been updated if necessary.
• The documentation is still up-to-date in the doc folder.
• The dependencies are still up-to-date in pyproject.toml.

[drewj-tp]

Thanks @john-science I don't have any major suggestions here. Some nits that I'm not going to hold as blockers for this PR