A helper to generate the example model

[RasmusSkytte]

Intent

This PR fixes a pain point whenever we modify the example model used to generate the data for DiseasySeirExample which we use -- among other things -- to test our modelling framework.

When we modify this model, we need to find all places in the tests that uses / modifies this model in their own tests and this is can be a rather annoying process.

Approach

A un-exported function generate_example_seir_model() was created to produce the model used in the example data.

This function has overrides to change parameters or modules to allow the tests to create small variations of this model in their tests.

Known issues

N/A

Checklist

• The PR passes all local unit tests
• I have documented any new features introduced
• If the PR adds a new feature, please add an entry in NEWS.md
• A reviewer is assigned to this PR

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 64.41%. Comparing base (6c1fcdd) to head (4c03a5b).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #290      +/-   ##
==========================================
+ Coverage   64.17%   64.41%   +0.23%     
==========================================
  Files          28       29       +1     
  Lines        9597     9661      +64     
==========================================
+ Hits         6159     6223      +64     
  Misses       3438     3438              

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