905 add age group resolution to seir and sir model

[johapau]

Changes and Information

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

• Integration of age groups into the easier models
• Examples demonstrating the usage

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[HenrZu]

Thanks, Good work 👍
We should test all examples and check if the results are still equal to the former version. I would recommend to use the print_table function from the time series class. Then you can simply write the ostream to a txt file and then compare :)
Also, a new example ode_seir_ageres.cpp would probably be nice

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.41%. Comparing base (a245aee) to head (4e874c9).
Report is 7 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #911      +/-   ##
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+ Coverage   96.34%   96.41%   +0.06%     
==========================================
  Files         129      130       +1     
  Lines       10072    10265     +193     
==========================================
+ Hits         9704     9897     +193     
  Misses        368      368              

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