Expand allowable XS types to include lower case letters

[mgjarrett]

What is the change?

Allow lower case letters to be used as the XS type to increase the number of possible unique XS groups from 26 to 52.

Why is the change being made?

Some models have a large number of different XS types (15-20), and some internal plugins generate additional XS groups while running, which can push the total number of XS groups above 26. The ARMI app will currently crash if a 27th XS type is requested.

Notes

This should work fine on Linux or Mac platforms, which have case-sensitive filesystems. A user might encounter problems if they attempt to exceed 26 XS types on Windows, because the filesystem is not case-sensitive, and a plugin might try to generate a file name that is indistinguishable to a previous filename on Windows (e.g., latticePhysics-AA.inp and then latticePhysics-aA.inp). While this is unfortunate, the current code would also fail when a user tries to exceed 26 XS types (on any platform), so this PR isn't breaking a currently working capability, it's just changing the manner in which the code would fail.

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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.