Lookup station information to add the station latitude and longitude to the DataFrame.
This function will add two columns to the DataFrame (‘latitude’ and ‘longitude’) after
looking up all unique station identifiers available in the DataFrame.
df (pandas.DataFrame) – The DataFrame that contains the station observations
stn_var (str) – The string of the variable name that represents the station in the DataFrame. Common
examples are ‘station’, ‘stid’, and ‘station_id’
pandas.DataFrame that contains original Dataframe now with the latitude and longitude
values for each location found in station_info.