refactor: Move map_population to DiseasyPopulation

[RasmusSkytte]

Intent

This PR is a stepping stone for implementing contact matrices per region.

Here we refactor the code to move the map_population from DiseasyActivity to DiseasyPopulation

Approach

map_population is moved without changes but some extra temporary changes and passing of variables had to be introduced in functions that depended on it.

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

❌ Patch coverage is 91.39785% with 8 lines in your changes missing coverage. Please review.
✅ Project coverage is 63.77%. Comparing base (00ab653) to head (f85eb91).

Files with missing lines	Patch %	Lines
R/DiseasyActivity.R	33.33%	4 Missing ⚠️
R/DiseasyPopulation.R	96.47%	3 Missing ⚠️
R/DiseasyModelOde.R	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #286      +/-   ##
==========================================
- Coverage   63.83%   63.77%   -0.07%     
==========================================
  Files          28       28              
  Lines        9585     9593       +8     
==========================================
- Hits         6119     6118       -1     
- Misses       3466     3475       +9     

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