Create limit order functionality

[silencesdscope]

• process() should output whether the order is market or limit
• if an order is a limit order it should be appended to openOrders list with the relevant variables
• those variables being: postype, num units, target price, limit order date (there may be more)
• before trades are initialised, you should iterate each openOrder and check if its target price is met (condition based on postype)
• if the target is met, use the info from the current data row and the order to append it to openPositions
• the code then continues and runs self.process()

This will lead to more conditionals inside the trade initialisation part of backtest() so you need to think of a cleaner way to do this than the above.