Composite Model - Add function return in output

[teubert]

Add returns of composite model function members to composite model output.

[github-actions[bot]]

Thank you for opening this PR. Each PR into dev requires a code review. For the code review, look at the following:

• [ ] Reviewer (someone other than author) should look for bugs, efficiency, readability, testing, and coverage in examples (if relevant).
• [ ] Ensure that each PR adding a new feature should include a test verifying that feature.
• [ ] All errors from static analysis must be resolved.
• [ ] Review the test coverage reports (if there is a change) - will be added as comment on PR if there is a change
• [ ] Review the software benchmarking results (if there is a change) - will be added as comment on PR
• [ ] Any added dependencies are included in requirements.txt, setup.py, and dev_guide.rst (this document)
• [ ] All warnings from static analysis must be reviewed and resolved - if deemed appropriate.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (ec48258) 83.29% compared to head (ec48258) 83.29%.

:exclamation: Current head ec48258 differs from pull request most recent head a367143. Consider uploading reports for the commit a367143 to get more accurate results

Additional details and impacted files

```diff @@ Coverage Diff @@ ## dev #139 +/- ## ======================================= Coverage 83.29% 83.29% ======================================= Files 100 100 Lines 10507 10507 ======================================= Hits 8752 8752 Misses 1755 1755 ```

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.