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 :white_check_mark:

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

:exclamation: 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

```diff @@ 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