Ensure FixedT and FixedP PTE solvers use sensible uscale

[jhp-lanl]

PR Summary

The FixedT and FixedP PTE solvers were adding the weighteed component energies from the initial density guess to generate a characteristic scale for the energy and pressure. However, this wasn't hardened against the possibility that the energies could be negative, which would cause the solver to diverge in the opposite direction of a solution.

This PR uses an absolute value of the energy for the FixedT solver and and absolute value of the input pressure for the FixedP solver.

This functionality is essentially tested in the MultiEOS MR, so I won't add a test here.

PR Checklist

• N/A 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.
• N/A 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

[Yurlungur]

Oof good catch. Thanks for the fix!