Removing long value of static member in class of test_composities fro…

[zachmprince]

What is the change?

This PR adds a doco meta field in reactor.tests.test_composities.MockBP that tells sphinx to not display the value of a couple static members.

Why is the change being made?

These members were essentially listing every nuclide in our library, which isn't very useful from a documentation point of view. It was also preventing a PDF build of the documentation.

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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 important changes.
• The documentation is still up-to-date in the doc folder.
• No requirements were altered.
• The dependencies are still up-to-date in pyproject.toml.

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