'Creates a Market order.
A market order is an order to buy or sell a stock at the best available price.
Generally, this type of order will be executed immediately. However, the price at which a market order will be executed
is not guaranteed.
:param action: The order action.
:type action: Order.Action.BUY, or Order.Action.BUY_TO_COVER, or Order.Action.SELL or Order.Action.SELL_SHORT.
:param instrument: Instrument identifier.
:type instrument: string.
:param quantity: Order quantity.
:type quantity: int/float.
:param onClose: True if the order should be filled as close to the closing price as possible (Market-On-Close order). Default is False.
:type onClose: boolean.
:rtype: A :class:`MarketOrder` subclass.'

@abc.abstractmethod
def createMarketOrder(self, action, instrument, quantity, onClose=False):

    raise NotImplementedError()
