'Open an existing ``rootdir`` for writing.
Parameters
end_session : Timestamp (optional)
When appending, the intended new ``end_session``.'

@classmethod
def open(cls, rootdir, end_session=None):

    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))
