Updating TD_frac for consistency

[keckler]

This does the same exact thing as #1114 . However, that PR was having some mysterious issues where the tests were passing locally, but not via GH actions. This mysterious behavior was isolated to a specific unit test that was previously implemented in its own file at materials/tests/test_uraniumOxide. For an unknown reason, that was causing the problems. Moving the test into materials/tests/test_materials seems to have fixed the problem.

Description

This closes #1112 by ensuring that the TD_frac material modification on UraniumOxide and MOX both actually change the density as they are supposed to.

This is implemented mainly via changes to the SimpleSolid class and by making setting a default value of 1.0 to the theoreticalDensityFrac material attribute.

Other than that, ThoriumOxide was modified to be consistent in its usage of the new theoreticalDensityFrac

This doesn't touch anything in the API, so users should not notice any changes except the fact that TD_frac is now correctly applied, so their results might change if they were using the framework oxide classes.

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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.