Reads the COMTRADE ID of a given channel number.
        The number to be given is the same of the COMTRADE header.

def getDigitalID(self,num):
        """
        Reads the COMTRADE ID of a given channel number.
        The number to be given is the same of the COMTRADE header.
        """
        listidx = self.Dn.index(num) # Get the position of the channel number.
        return self.Dch_id[listidx]