code: def humidity_series(self):
          
          return [(tstamp, \
                  self._station_history.get_measurements()[tstamp]['humidity']) \
                  for tstamp in self._station_history.get_measurements()]
docs: Returns the humidity time series relative to the meteostation, in
          the form of a list of tuples, each one containing the couple
          timestamp-value
  
          :returns: a list of tuples