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.
modify limiting case
Add test for amortizing bond
fix denominator
Inline template-method specializations.

This avoids double-definition errors on some platforms.
Merge latest changes from master.
Add new files to build.
Add missing inclusion.
Merge pull request #158.