Adding code coverage to FIXSRC code

[john-science]

What is the change?

Adding code coverage to FIXSRC code

Why is the change being made?

The DIF3D file format is important, and we have never had any unit tests for it.

Thanks to Aaron, for giving me the reference material I needed to understand the code.

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Checklist

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

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

• The release notes have been updated if necessary.
• The documentation is still up-to-date in the doc folder.
• The dependencies are still up-to-date in pyproject.toml.

[albeanth]

Just curious, is the code here a complete feature? Or does it need more to actually do something useful? Specifically the fixsrc.py file.

[john-science]

Just curious, is the code here a complete feature? Or does it need more to actually do something useful? Specifically the fixsrc.py file.

The actual code ARMI has to read and write FIXSRC files appears to be fully functional. Aaron gave me some test files (that are WAY too big to commit to the repo), and I was able to test code to my heart's content. But this isn't surprising, considering we actually used this code to drive DIF3D for years.