#269 Make UnsupportedPredicateException public.

• Very small follow-up to !814 (merged). I forgot to make UnsupportedPredicateException public, and it is a checked exception thrown by some of the methods I made public in CifToBddConverter. When calling them, this exception must be caught. But since it is private, that code won't compile. So, make this public as well.

Addresses #269