minor improve in trading.env

[PipsHustler2017]

To be precise, I guess self._current_tick and self._last_trade_tick shall start from self.window_size instead of None.

        # episode
        self._start_tick = self.window_size
        self._end_tick = len(self.prices) - 1
        self._done = None
        self._current_tick = self.window_size  # None
        self._last_trade_tick = self.window_size  # None

[PipsHustler2017]

My understanding is wrong after running more trials. Close here.