Initialise the main dataframe for the results and the custom timers dataframes

def _init_dataframes(self):
        """Initialise the main dataframe for the results and the custom timers dataframes
        """
        df = pd.read_sql_query("SELECT elapsed, epoch, scriptrun_time, custom_timers FROM result ORDER BY epoch ASC",
                               db.get_conn())

        self._get_all_timers(df)
        self.main_results = self._get_processed_dataframe(df)

        # create all custom timers dataframes
        for key, value in six.iteritems(self._timers_values):
            df = pd.DataFrame(value, columns=['epoch', 'scriptrun_time'])
            df.index = pd.to_datetime(df['epoch'], unit='s')
            timer_results = self._get_processed_dataframe(df)
            self.timers_results[key] = timer_results

        # clear memory
        del self._timers_values