Add highway search to pbessolve/pbes2bool (version from November 2019)

[jgroote]

The classical pbes2bool had a form of highway search where the number of simultaneously searched BES variables would be limited by setting the excess variables to true or false. The new version of pbes2bool, which is a copy of pbessolve, does not have highway search available. This extension request is to make a form of highway search available for the new version of pbes2bool.