Set commission (transaction fee) function.

        Args:
            fn (fn(quantity, price)): Function used to determine commission
            amount.

def set_commissions(self, fn):
        """
        Set commission (transaction fee) function.

        Args:
            fn (fn(quantity, price)): Function used to determine commission
            amount.

        """
        self.commission_fn = fn

        for c in self._childrenv:
            if isinstance(c, StrategyBase):
                c.set_commissions(fn)