1386 implement influenza model

[HenrZu]

Changes and Information

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

• Implement influenza model from doi.org/10.1186/s12879-017-2344-6 into our framework
• Add flowchart and rtd documentation

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

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• Every addressed issue is linked (use the "Closes #ISSUE" keyword below)
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• Tests are added for new functionality and a local test run was successful (with and without OpenMP)
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• (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
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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.29%. Comparing base (a1d8af8) to head (9fd5e08).
⚠️ Report is 9 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff            @@
##             main    #1387    +/-   ##
========================================
  Coverage   97.28%   97.29%            
========================================
  Files         175      178     +3     
  Lines       15306    15635   +329     
========================================
+ Hits        14891    15212   +321     
- Misses        415      423     +8     

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[annawendler]

Looks really good! :) Most of my comments are regarding naming of variables/parameters where I made some suggestions.

[annawendler]

There are several conversations that you resolved without making any changes or commenting. Can you add comments where you disagree so we can discuss?

[annawendler]

Some of the parameter names need to be updated.

[annawendler]

Suggested change

	* The destinction into the 2 layers of susceptibility is done via the parameters.
	* The distinction into the 2 layers of susceptibility is done via the parameters.

[annawendler]

I would prefer two separate parameters for the transitions and then setting it equal in the example since it allows more flexibility for the user.