1315 support IO functionality with single age group for ODE SECIR

[HenrZu]

Changes and Information

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

• when inititialize the ODE SECIR model with real data, we were restricted to a model with 6 age groups (using the official reported data)
• Now, we can also input a model with a single age group with correct initialization.
• Short Fix for readthedocs build.

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

• 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

❌ Patch coverage is 97.82609% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 97.26%. Comparing base (e12b5a1) to head (a3cc1dd).
⚠️ Report is 5 commits behind head on main.

Files with missing lines	Patch %	Lines
cpp/models/ode_secir/parameters_io.h	97.82%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1354      +/-   ##
==========================================
- Coverage   97.34%   97.26%   -0.09%     
==========================================
  Files         174      174              
  Lines       15098    15153      +55     
==========================================
+ Hits        14697    14738      +41     
- Misses        401      415      +14     

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[charlie0614]

Looks good. See the minor questions.