code: def position_pnl(self):
          
          last_price = self._data_proxy.get_last_price(self._order_book_id)
          if self._direction == POSITION_DIRECTION.LONG:
              price_spread = last_price - self._last_price
          else:
              price_spread = self._last_price - last_price
  
          return self._logical_old_quantity * self._contract_multiplier * price_spread
docs: [float] 昨仓盈亏，策略在当前交易日产生的盈亏中来源于昨仓的部分