Dump all results to CSV.

def save_csv(self):
        """ Dump all results to CSV. """
        # Sort results so we can start to see patterns right in the raw CSV.
        self.results.sort_values(by=self.column_ids, inplace=True)
        # Gotcha: integers seems to be promoted to float64 because of
        # reindexation. See: https://pandas.pydata.org/pandas-docs/stable
        # /gotchas.html#na-type-promotions
        self.results.reindex(columns=self.column_ids).to_csv(
            self.csv_filepath, index=False)