sum_visits() summarizes the number of visits by person within a timeframe,
and optionally by visit_class of visit.
sum_visits(wt, timeframe = NULL, visit_class = NULL)a data.frame with columns panelist_id, column indicating the time unit
(unless timeframe set to NULL), n_visits indicating the number of visits,
and a column for each value of visit_class, if specified.
webtrack data object.
character. Indicates for what time frame to aggregate visits.
Possible values are "date", "week", "month", "year", "wave" or NULL.
If set to "wave", wt must contain a column call wave. Defaults to NULL,
in which case the output contains number of visits for the entire time.
character. Column that contains a classification of visits.
For each value in this column, the output will have a column indicating the
number of visits belonging to that value. Defaults to NULL.