Expose entropy to python api

[AstroBarker]

Extended code to expose the entropy functionality to the Python API.
Added two additional tests in python_bindings.py that check the entropy functionality in the [Vector EOS] bits.
The Python binding stuff was a bit new to me, so I may well have missed something. It does work for the IdealGas and StellarCollapse models as expected, though.

• 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
• If preparing for a new release, update the version in cmake.

[Yurlungur]

Thanks for adding this!

[jhp-lanl]

@rbberger could you take a look?

[rbberger]

Looks ok, although nothing actually sets entropy in EOSState.

[Yurlungur]

Updated to main. Will merge if tests pass on re-git

[Yurlungur]

@AstroBarker this PR seems to fail when run on re-git with the full test buildout. Did you ever build the full python machinery when you tested?