fix interpolation of mass fractions

[Yurlungur]

PR Summary

The Stellar Collapse EOS had a bug where the mass fraction interpolation had two indices swapped. This quick trivial fix remedies the issue.

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