Clears prices according to auction type.

def _clearPrices(self):
        """ Clears prices according to auction type.
        """
        for offbid in self.offers + self.bids:
            if self.auctionType == DISCRIMINATIVE:
                offbid.clearedPrice = offbid.price
            elif self.auctionType == FIRST_PRICE:
                offbid.clearedPrice = offbid.lmbda
            else:
                raise ValueError