Opens an existing data file to append to

        :param fname: File path of the location for the data file to open
        :type fname: str

def load_data_file(self, fname, filemode='a'):
        """Opens an existing data file to append to

        :param fname: File path of the location for the data file to open
        :type fname: str
        """
        self.close_data()
        self.datafile = open_acqdata(fname, filemode=filemode)

        self.explorer.set(datafile=self.datafile)
        self.protocoler.set(datafile=self.datafile)
        self.charter.set(datafile=self.datafile)
        self.bs_calibrator.set(datafile=self.datafile)
        self.tone_calibrator.set(datafile=self.datafile)
        self.set_calibration(None)

        self.current_cellid = dict(self.datafile.get_info('')).get('total cells', 0)