Append columns with ∆T between rows to data.

def append_delta_index(self, ts_data=None, data_delta=None, key=KEY_DATA):
        """Append columns with ∆T between rows to data."""
        reasign = False
        if data_delta is None:
            if self.data is not None:
                data_delta = self.data[key]
            else:
                self.printif('NO HAY DATOS PARA AÑADIR DELTA', 'error')
                return None
            reasign = True
        data_delta['delta'] = data_delta.index.tz_convert('UTC')
        if ts_data is not None:
            data_delta['delta'] = (data_delta['delta'] - data_delta['delta'].shift(1)).fillna(ts_data)
            data_delta['delta_T'] = data_delta['delta'].apply(lambda x: pd.Timedelta(x).seconds) / ts_data
        else:
            data_delta['delta'] = (data_delta['delta'] - data_delta['delta'].shift(1)).fillna(0)
        if reasign:
            self.data[key] = data_delta
        else:
            return data_delta