TD_frac updates and consistency

[keckler]

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 parameter.

Other than that, the other material classes that were originally using some form of a theoreticalDensityFrac variable were modified to be consistent in their usage of that variable. Those other material classes included ThoriumOxide and B4C.

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.

EDIT: This PR no longer makes any changes to B4C. A new ticket was opened to address that class.

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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.

[keckler]

I am testing our internal projects against this change now, and will update when I know that those work.

[keckler]

This appears to be working. I had to make one change, to the internal version of B4C. But that was easy to do, and now this should be ready to land.

[keckler]

I see something is failing... grrr

[keckler]

Okay @john-science , this is ready for review.

[keckler]

And now I need to change this to account for #1062

[keckler]

I have no idea why these tests are failing. They all pass locally, and looking at the ones that claim to be failing, they seem pretty random...

[keckler]

@john-science Have you seen this before? The tests passed with Python3.9 but not 3.10...

[ntouran]

would be good to update the user docs to explain this in the materials docs.