Returns a standard set of pandas.DataFrames:
        equities, futures, exchanges, root_symbols

def _load_data(self,
                   equities,
                   futures,
                   exchanges,
                   root_symbols,
                   equity_supplementary_mappings):
        """
        Returns a standard set of pandas.DataFrames:
        equities, futures, exchanges, root_symbols
        """
        # Set named identifier columns as indices, if provided.
        _normalize_index_columns_in_place(
            equities=equities,
            equity_supplementary_mappings=equity_supplementary_mappings,
            futures=futures,
            exchanges=exchanges,
            root_symbols=root_symbols,
        )

        futures_output = self._normalize_futures(futures)

        equity_supplementary_mappings_output = (
            self._normalize_equity_supplementary_mappings(
                equity_supplementary_mappings,
            )
        )

        exchanges_output = _generate_output_dataframe(
            data_subset=exchanges,
            defaults=_exchanges_defaults,
        )

        equities_output, equities_mappings = self._normalize_equities(
            equities,
            exchanges_output,
        )

        root_symbols_output = _generate_output_dataframe(
            data_subset=root_symbols,
            defaults=_root_symbols_defaults,
        )

        return AssetData(
            equities=equities_output,
            equities_mappings=equities_mappings,
            futures=futures_output,
            exchanges=exchanges_output,
            root_symbols=root_symbols_output,
            equity_supplementary_mappings=equity_supplementary_mappings_output,
        )