clean up SAT.UnsatBasedWBO module
clean up SAT.PBO module a bit
provide Unsat-based PBO functionality through SAT.PBO module
implement msu4-like algorithm for PBO/WBO

Joao P. Marques-Silva and Jordi Planes.
Algorithms for Maximum Satisfiability using Unsatisfiable Cores.
In Design, Automation and Test in Europe, 2008 (DATE '08). March 2008.
pp. 408-413, doi:10.1109/date.2008.4484715.