Open an existing ``rootdir`` for writing.

        Parameters
        ----------
        end_session : Timestamp (optional)
            When appending, the intended new ``end_session``.

def open(cls, rootdir, end_session=None):
        """
        Open an existing ``rootdir`` for writing.

        Parameters
        ----------
        end_session : Timestamp (optional)
            When appending, the intended new ``end_session``.
        """
        metadata = BcolzMinuteBarMetadata.read(rootdir)
        return BcolzMinuteBarWriter(
            rootdir,
            metadata.calendar,
            metadata.start_session,
            end_session if end_session is not None else metadata.end_session,
            metadata.minutes_per_day,
            metadata.default_ohlc_ratio,
            metadata.ohlc_ratios_per_sid,
            write_metadata=end_session is not None
        )