610 implement initialization scheme for flows

[lenaploetzke]

Changes and Information

Please briefly list the changes (main added features, changed items, or corrected bugs) made:

• Added functionality to calculate initial flows based on real data for an IDE-SECIR model.
• Added functionality to get mean values to StateAgeFunction.
• Added tests and an example.

If need be, add additional information and what the reviewer should look out for in particular: -/-

Merge Request - Guideline Checklist

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Checks by code author

• [x] Every addressed issue is linked (use the "Closes #ISSUE" keyword below)
• [x] New code adheres to coding guidelines
• [x] No large data files have been added (files should in sum not exceed 100 KB, avoid PDFs, Word docs, etc.)
• [x] Tests are added for new functionality and a local test run was successful (with and without OpenMP)
• [x] Appropriate documentation for new functionality has been added (Doxygen in the code and Markdown files if necessary)
• [x] Proper attention to licenses, especially no new third-party software with conflicting license has been added
• [ ] (For ABM development) Checked benchmark results and ran and posted a local test above from before and after development to ensure performance is monitored.

Checks by code reviewer(s)

• [x] Corresponding issue(s) is/are linked and addressed
• [x] Code is clean of development artifacts (no deactivated or commented code lines, no debugging printouts, etc.)
• [x] Appropriate unit tests have been added, CI passes, code coverage and performance is acceptable (did not decrease)
• [x] No large data files added in the whole history of commits(files should in sum not exceed 100 KB, avoid PDFs, Word docs, etc.)
• [x] On merge, add 2-5 lines with the changes (main added features, changed items, or corrected bugs) to the merge-commit-message. This can be taken from the briefly-list-the-changes above (best case) or the separate commit messages (worst case).

Closes #610.

[codecov[bot]]

Codecov Report

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

Project coverage is 96.39%. Comparing base (8d0d615) to head (533b624).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #952 +/- ## ========================================== + Coverage 96.34% 96.39% +0.04% ========================================== Files 129 130 +1 Lines 10071 10209 +138 ========================================== + Hits 9703 9841 +138 Misses 368 368 ```

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[annawendler]

Note: Once this PR is merged the links will work in PR #1000 and it can be merged.