simplify 'area_per_resident' estimate

[mpadge]

No description provided.

[codecov]

Codecov Report

❌ Patch coverage is 68.53933% with 28 lines in your changes missing coverage. Please review.
✅ Project coverage is 86.56%. Comparing base (1440c13) to head (182cc7e).
⚠️ Report is 14 commits behind head on main.

Files with missing lines	Patch %	Lines
R/population-estimate.R	0.00%	28 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #11      +/-   ##
==========================================
- Coverage   86.65%   86.56%   -0.09%     
==========================================
  Files          14       14              
  Lines        1064     1124      +60     
==========================================
+ Hits          922      973      +51     
- Misses        142      151       +9     

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