Fix a bug when the coupon rate is very near 0

Original implementation of sinkingNotionals will raise singularity when coupon is very near 0. I suggest to add some threshold check on the coupon rate and when it is low change to limitation case.
I think the denominator should be just `nPeriods` here. Am I correct?
(For instance, if `nPeriods` is 2 and the initial notional is 100 the resulting notionals should be [100, 100/2, 0]. Come to think of it, it would be nice to have a couple of test cases for this function...)