This method calculates the difference in seconds between
            the 2 columns (column2 - column1)

        The final result may provided negative values depending on the values
        from column1 and column2.

        :param column1: first column. Values in column1 must be datetime type
        :param column2: second column. Values in column2 must be datetime type
        :type column1: string
        :type column2: string

        :return: original dataframe with a new column with the difference
            between column2 - column1
        :rtype: pandas.DataFrame

def enrich(self, column1, column2):
        """ This method calculates the difference in seconds between
            the 2 columns (column2 - column1)

        The final result may provided negative values depending on the values
        from column1 and column2.

        :param column1: first column. Values in column1 must be datetime type
        :param column2: second column. Values in column2 must be datetime type
        :type column1: string
        :type column2: string

        :return: original dataframe with a new column with the difference
            between column2 - column1
        :rtype: pandas.DataFrame
        """

        if column1 not in self.data.columns or \
           column2 not in self.data.columns:
            return self.data

        self.data["timedifference"] = (self.data[column2] - self.data[column1]) / np.timedelta64(1, 's')
        return self.data