Fix Pu frac calculation

[mgjarrett]

Description

The previous puFrac calculation had a numerator that was adjusted for density changes (from block expansion) but a constant denominator that did not account for expansion.

The new formula calculates number of Pu atoms directly and does not account for density in either the numerator or denominator.

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Checklist

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

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

• The release notes (location doc/release/0.X.rst) are up-to-date with any bug fixes or new features.
• The documentation is still up-to-date in the doc folder.
• The dependencies are still up-to-date in setup.py.

[opotowsky]

Checkout that test!

Thanks, and this looks good to me! Confirmed to work in practice, as well.