Allow users to define flag names with numerical values

[john-science]

What is the change?

Historically, ARMI Flags have been all-caps and English Alphabet only: FUEL, DUCT``, FEED`, and so on. And if you added a numerical digit into the mix, it would be ignored (for various reasons).

This PR makes it so if the user defines a parameter it will be treated as an exception to the "letters only" rule, and handled exactly as-is.

This is only meant to support "upper case letters and numbers though". This PR still does not support flag names with:

• whitespace
• non-English characters
• special characters, like punctuation
• very long names

Why is the change being made?

This was a feature request made by downstream users.

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Checklist

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

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

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