Avoiding rates to discount calculation where possible

[aleCombi]

Rate stream calculation often go between the space of rates and the space of accruals (e.g.: rate -> 1 + rate*time) or vice versa. This should be done the least number of times possible.

For example, calculating a discount factors ratio, to then get a forward rate, to then calculate an expected forward flow is a waste of computation, unless discrepancies between day_counts or similar details exist.

[aleCombi]

Even worse is the case of compounded rates, where this happens once for the underlying rate and once for the compounded one.