Reorganizing Math Utilities

[john-science]

Description

This is just a code reorg to move a lot of math-specific utilities from armi/utils/__init__.py to a more predictable location in armi/utils/mathematics.py.

I put in some imports so this won't break backwards compatibility.

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Checklist

• The code is understandable and maintainable to people beyond the author.
• Tests have been added/updated to verify that the new or changed code works.
• There is no commented out code in this PR.
• The commit message follows good practices.
• All docstrings are still up-to-date with these changes.

[ntouran]

good update. thanks!