Improve the handling of age.var and id.vars within charlson comorbidities

[dewittpe]

Prior to this change when Charlson comorbidities were requested and id.vars was NULL and age.var was not NULL then there would be an error as the merge to add the age scores onto the results. When there was more than one age value for a unique id.var, the return will have more than one row for the id.var. This is conceptually bad.

This change adds to tests/test-comorbidites.R to check for warning messages and expected returns when the id.vars are not one-to-one with c(id.vars, age.var) These changes make sure the error does not occur. Warnings are thrown when there is more than one age for the id.vars

fix #43

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.57%. Comparing base (517bb53) to head (b76a89d).
⚠️ Report is 6 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #44      +/-   ##
==========================================
+ Coverage   96.54%   96.57%   +0.02%     
==========================================
  Files          17       17              
  Lines        1304     1314      +10     
==========================================
+ Hits         1259     1269      +10     
  Misses         45       45              

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