Add a POSIXct date column as well as a column with the appropreiate granularity to a World Bank API return
wbdate2POSIXct(df, date_col)
data frame returned from API call
name of the current date field
If the package lubridate (>= 1.5.0) is available the original data frame with two new columns,
data_ct
and granularity
is returned. If the above package is not available,
the orignal data frame is returned unaltered with an additional warning message.