Allow setting multi-universe fills

[MicahGale]

Pull Request Checklist for MontePy

Description

This fixes a "bug" where setting Fill.universes was not properly exported. There was previously... an effort to implement this, which was not successful. This changed:

1. Making sure min_index and max_index is always accessible without crashing
2. Setting min_index and max_index whenever universes is set.
3. setting mulitple_universes=True whenever universes is set.
4. _update_cell_values works properly.

Fixes #714

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General Checklist

• I have performed a self-review of my own code.
• The code follows the standards outlined in the development documentation.
• I have formatted my code with black version 25.
• I have added tests that prove my fix is effective or that my feature works (if applicable).

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Documentation Checklist

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• For infrastructure updates, I have updated the developer's guide.
• For significant new features, I have added a section to the getting started guide.

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Ensure that:

• The submitted code is consistent with the merge checklist outlined here.
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• 100% coverage of the patch is achieved, or justification for a variance is given.

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📚 Documentation preview 📚: https://montepy--717.org.readthedocs.build/en/717/