Fix bug in FixedT PTE solver

[jhp-lanl]

There was a small indexing bug in the FixedT PTE solver with an index going out of bounds. The residual has size nmat in this PTE solver so the last iteration was OOB.

PR Summary

PR Checklist

• Adds a test for any bugs fixed. Adds tests for new features.
• Format your changes by using the make format command after configuring with cmake.
• Document any new features, update documentation for changes made.
• Make sure the copyright notice on any files you modified is up to date.
• After creating a pull request, note it in the CHANGELOG.md file.
• LANL employees: make sure tests pass both on the github CI and on the Darwin CI

If preparing for a new release, in addition please check the following:

• Update the version in cmake.
• Move the changes in the CHANGELOG.md file under a new header for the new release, and reset the categories.
• Ensure that any when='@main' dependencies are updated to the release version in the package.py

[jhp-lanl]

I'm not going to add a FixedT test right now, but I think this should be tested better with my MultiEOS MR coming up

[Yurlungur]

Thanks for the fix!

[Yurlungur]

Tests passing on re-git. Merging.