Dates in history are not neccessarily ordered

[maxkoe]

When adding data to the history, date rows are added on the fly. This can lead to the dates being not well ordered. It would be preferable if they are ordered at all times.

Minimal example

import pandas as pd
import numpy as np

from zipline.history.history import HistorySpec
from zipline.protocol import BarData
from zipline.finance.trading import TradingEnvironment

from gg.powerline.history.history_container import EpexExchange
from gg.powerline.history.history_container import EpexHistoryContainer

start_date = pd.Timestamp('2015-06-25', tz='Europe/Berlin').tz_convert('UTC')
end_date = pd.Timestamp('2015-06-28', tz='Europe/Berlin').tz_convert('UTC')
source_start = start_date - pd.Timedelta(hours=2)
exchange = EpexExchange(start=start_date, end=end_date)
env = exchange.env
asset_metadata = {}
env.write_data(futures_data=asset_metadata)
history_spec = HistorySpec(bar_count=2, frequency='1m', field='price',
                           ffill=False, data_frequency='minute', env=env)
history_specs = {}
history_specs[history_spec.key_str] = history_spec
container = EpexHistoryContainer(history_specs, None, source_start, 'minute',
                                 env=env)

data = {}
data[1] = {'dt': pd.Timestamp('2015-06-27'),
           'price': np.random.uniform(0, 100),
           'market': 'epex_auction',
           'product': '00-01',
           'day': pd.Timestamp('2015-06-27'),
           'sid': 1}
data[2] = {'dt': pd.Timestamp('2015-06-26'),
           'price': np.random.uniform(0, 100),
           'market': 'epex_auction',
           'product': '00-01',
           'day': pd.Timestamp('2015-06-26'),
           'sid': 2}
for current_sid in data:
    current_data = data[current_sid]
    bar = BarData({current_sid: current_data})
    container.update(bar, current_data['dt'])

history = container.get_history()

print(history['epex_auction'])