Expose Gibbs free energy

[Yurlungur]

PR Summary

This PR exposes the Gibbs free energy. I do this by transforming from the entropy in the base class. But of course, we could instead do the opposite. Individual EOS's may override the defaults but I only implemented the defaults in the base class and variant. Down the line we will need to expose proper passthroughs for EOSPAC and the tabulated EOS's. I also add missing documentation for entropy and free energy. This MR resolves #397 .

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]

Thanks for the useful feedback, @aematts ! I addressed your comments. Can you re-review and approve at your convenience?

[aematts]

Go!

[jhp-lanl]

Thanks @Yurlungur ! This looks good to me. Thanks for adding the note about EOSPAC