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
        ()

def whereis(self, channel):
        """ 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
        ()

        """
        if channel in self:
            return tuple(self.channels_db[channel])
        else:
            return tuple()