Matching the Magnesium density curve to match the reference

[john-science]

Description

Based on a discussion here with @dpham-materials, it appears our Magnesium density curve very slightly mis-matched the provided reference.

It is a very slight difference, but since there is no other reference provided, I am just going to fix it.

@opotowsky It will be up to us to determine if any downstream projects/integration tests actually USE this material. If not, it's easy. If so, we will have to determine how this change affects our project results.

This PR is part of this umbrella ticket.

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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]

I can kick off tests internally

[opotowsky]

This has no downstream impacts as far as my testing shows.