1392 update IDE-SECIR examples and improve some documentation

[annawendler]

Changes and Information

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

• Merge examples ide_secir and ide_secir_ageres to avoid code duplication
• Add documentation in IDE-SECIR examples, for implemented formulas in general and in get_mean() of StateAgeFunction

Merge Request - Guideline Checklist

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Checks by code author

• Every addressed issue is linked (use the "Closes #ISSUE" keyword below)
• New code adheres to coding guidelines
• No large data files have been added (files should in sum not exceed 100 KB, avoid PDFs, Word docs, etc.)
• Tests are added for new functionality and a local test run was successful (with and without OpenMP)
• Appropriate documentation within the code (Doxygen) for new functionality has been added in the code
• Appropriate external documentation (ReadTheDocs) for new functionality has been added to the online documentation
• 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)

• Corresponding issue(s) is/are linked and addressed
• Code is clean of development artifacts (no deactivated or commented code lines, no debugging printouts, etc.)
• Appropriate unit tests have been added, CI passes, code coverage and performance is acceptable (did not decrease)
• No large data files added in the whole history of commits(files should in sum not exceed 100 KB, avoid PDFs, Word docs, etc.)
• 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).

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.28%. Comparing base (52ed3c1) to head (2352ea2).
⚠️ Report is 10 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff            @@
##             main    #1402    +/-   ##
========================================
  Coverage   97.27%   97.28%            
========================================
  Files         175      180     +5     
  Lines       15306    15642   +336     
========================================
+ Hits        14889    15217   +328     
- Misses        417      425     +8     

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