code: def _load_next(self):
          
          if self._load_by_date:
              next_date = self.date + pds.DateOffset(days=1)
              return self._load_data(date=next_date)
          else:
              return self._load_data(fid=self._fid+1)
docs: Load the next days data (or file) without incrementing the date.
          Repeated calls will not advance date/file and will produce the same data
          
          Uses info stored in object to either increment the date, 
          or the file. Looks for self._load_by_date flag.