This function checks the validity and requirements for the highest_imputation argument.
It ensures that necessary conditions for date_imputation, min_dates,
and max_dates are met when highest_imputation is set to "Y".
assert_highest_imputation(
  highest_imputation,
  highest_imputation_values,
  date_imputation = NULL,
  max_dates,
  min_dates
)Returns NULL invisibly if assertions pass.
A character scalar indicating the highest level of imputation.
none
A character vector of valid values for highest_imputation.
none
Optional character scalar specifying the imputation method for dates.
NULL
Optional vector specifying maximum dates for imputation.
none
Optional vector specifying minimum dates for imputation.
none
If highest_imputation is "Y", either min_dates or max_dates must be specified.
If highest_imputation is "Y" and date_imputation is "first",
min_dates must be specified.
If highest_imputation is "Y" and date_imputation is "last",
max_dates must be specified.