Release 1.6.0

Merged IPASIR-UP theory propagation interface of Katalin Fazekas (@kfazekas) from SAT'23 tool paper:

Katalin Fazekas, Aina Niemetz, Mathias Preiner, Markus Kirchweger, Stefan Szeider and Armin Biere.
IPASIR-UP: User Propagators for CDCL. In Proceedings 26th International Conference on Theory and
Applications of Satisfiability Testing, July 4-8, Alghero, Italy, published by LIPIcs, 2023.