Refactor sim functions

[jpavlich]

• Changed age_mean_f to age in sim functions
• Added validation in get_sim_n_seropositive that checks same number of rows for foi and sample_size_by_age

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 70.37%. Comparing base (79e113a) to head (ce9b72a).

❗ Current head ce9b72a differs from pull request most recent head 1c9136d. Consider uploading reports for the commit 1c9136d to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #168      +/-   ##
==========================================
+ Coverage   70.32%   70.37%   +0.05%     
==========================================
  Files          10       10              
  Lines        1776     1779       +3     
==========================================
+ Hits         1249     1252       +3     
  Misses        527      527              

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[jpavlich]

Fixes #151