code: def whereis(self, channel):
          
          if channel in self:
              return tuple(self.channels_db[channel])
          else:
              return tuple()
docs: get ocurrences of channel name in the file
  
          Parameters
          ----------
          channel : str
              channel name string
  
          Returns
          -------
          ocurrences : tuple
  
  
          Examples
          --------
          >>> mdf = MDF(file_name)
          >>> mdf.whereis('VehicleSpeed') # "VehicleSpeed" exists in the file
          ((1, 2), (2, 4))
          >>> mdf.whereis('VehicleSPD') # "VehicleSPD" doesn't exist in the file
          ()