Discount date

[MarcusRainbow]

At present, the price returned by all pricers is discounted to the settlement date of the instrument being priced. This means that equities value at spot, listed options value at their screen price etc.

This matches the accounting policy in many banks (though not Goldman Sachs) but it is not really consistent. To be properly consistent, the price should be discounted to the same date, for all instruments. This means that not all instruments can value at their spot price.

We should also support pricing to a consistent discount date. This means supplying an optional discount date to pricer.price, which can use the credit id of contained instruments, plus their settlements, to work out how to discount the price to the supplied date.