ODFF: YIELD function

The only general way to implement this function is by iteration. 
Excel and Calc do 100 iterations. Thus ODFF has to set some 
realistic error limits - plus/minus epsilon is unrealistic.

The function is only calculating 'yield', and surely no-one needs 
more than a very few decimal places of accuracy - let's face it, 
by the time someone has calculated the yield, the price could 
have changed on the dealer's screen.

Incidentally Calc's algorithm is quite possibly superior to 
Excel's (tbc).

The description itself needs to mention iteration and the PRICE 
function in my view.

D