Retroactive fix of par contract calculation

Around v2.8.1. it was discovered that the par contract calculation in some earlier releases was not always correct or complete. This affected about 0.4% of the cases. The par value calculation seems correct.

Since v2.5.2 was an important release and some users rely on it, we decide to publish a retroactive fix. This is a "dead end" for the code development, though, so please consider moving to later versions if you can. As of this writing, the most recent version is v2.8.1.