Add support for indirect commodity quoting

[punchdeerflyscorpion]

Hi,
GnuCash supports indirect commodity quoting. For example, if the price database contains the valuation of a hypothetical instrument X in EUR and the valuation of EUR to USD, then gnucash is able to provide the value of instrument X in USD. Currently Piecash raises the GncConversionError (in Commodity.currency_conversion) if there is no direct quote in price database.

I am currently using the following modification to the currency_conversion method:

def currency_conversion(self, currency):
        """
        Return the latest conversion factor to convert self to currency

        Attributes:
            currency (:class:`piecash.core.commodity.Commodity`): the currency to which the Price need to be converted

        Returns:
            a Decimal that can be multiplied by an amount expressed in self.commodity to get an amount expressed in currency

        Raises:
            GncConversionError: not possible to convert self to the currency

        """
        # conversion is done from self.commodity to commodity (if possible)
        sc2c = self.prices.filter_by(currency=currency).order_by(Price.date.desc()).first()
        if sc2c:
            return sc2c.value

        # conversion is done directly from commodity to self.commodity (if possible)
        c2sc = currency.prices.filter_by(currency=self).order_by(Price.date.desc()).first()
        if c2sc:
            return Decimal(1) / c2sc.value

        **"""
        Indirect conversion workaround
        """

	priced_in = []
        ind = 0
        for pr in self.prices:
            if pr.currency not in priced_in:
                priced_in.append(pr.currency)
        for curr in priced_in:
            ind = self.currency_conversion(curr) * curr.currency_conversion(currency)
            if ind:
                return ind

        """
        End workaround
        """**

        raise GncConversionError("Cannot convert {} to {}".format(self, currency))