Create function to calculate patient counts as returned by tern::s_count_occurrences(), tern::s_num_patients(), and tern::s_count_occurrences_by_grade().

The function should be a wrapper for ard_categorical() that uses an id parameter to count patients instead of events, with the possibility to count by grade (i.e. for ordered factor variables) such that only the highest grade level is recorded for each patient.

This is currently somewhat implemented in ard_hierarchical() but only makes sense for hierarchies with >1 variables.

See https://insightsengineering.github.io/tern/latest-tag/reference/count_occurrences.html for details on the tern function.