add unitsystem test for DensityEnergyFromPressureTemperature

[Yurlungur]

PR Summary

This is motivated by some experimentation by @buechlerm in singularity-eos. I wanted to double check that DensityEnergyFromPressureTemperature is working properly. This adds a relevant test to ensure that's the case.

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

[Yurlungur]

Note this MR also changes the initialization from base units to match the expected convention.