realtime_add_local_datetime: Add local datetime column to realtime tibble
Description
Adds local_datetime and tz_used columns based on either the most common timezone in the original data or
a user supplied timezone. This function is meant to used in a pipe with the realtime_dd() function.
Date from realtime_dd is supplied in UTC which is the easiest format to work with across timezones. This function
does not change Date from UTC. Rather station_tz specifies the local timezone name and is useful in instances where
realtime_add_local_datetime adjusts local_datetime to a common timezone that is not the station_tz. This function is most
useful when all stations exist within the same timezone.