143 fix prepare serodata

[ntorresd]

This PR fixes input data validation for prepare_serodata and updates its documentation accordingly.

Error messages added or updated:

• "serodata must contain the right columns.". Now the error message specifies which of the mandatory columns is(are) missing.
• "serodata must contain both 'age_min' and 'age_max, or 'age_mean_f' to fully identify the age groups".

Warnings added when optional columns "country", "test" or "antibody" are missing. Assign to "None" when missing to avoid conflicts

[codecov-commenter]

Codecov Report

Attention: 21 lines in your changes are missing coverage. Please review.

Comparison is base (99fc72a) 76.04% compared to head (102cb97) 69.94%.

:exclamation: Current head 102cb97 differs from pull request most recent head 0e83d1d. Consider uploading reports for the commit 0e83d1d to get more accurate results

Files	Patch %	Lines
R/seroprevalence_data.R	48.78%	21 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #144 +/- ## ========================================== - Coverage 76.04% 69.94% -6.10% ========================================== Files 10 10 Lines 1603 1757 +154 ========================================== + Hits 1219 1229 +10 - Misses 384 528 +144 ```

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.