missing_combinations: Detect dates and hours missing in filenames
Description
Facebook mobility data is reported daily at 3 different hours
(0, 8, 16).
This function reads the data extracted from the current files and detects
if any file for a particular day or hour is missing.
A data.frame with the missing days and hours, if any.
Arguments
data
A data.frame with one column for the raster's date
and another for the hour. If not explicitly specified
in the function's arguments, the column names are day and hour.