PQuAcq and GQuAcq fixes

[Dimosts]

Minor (but important) fixes on GQuAcq and PQuAcq to work properly!

[Dimosts]

Needs to be merged first, before the other PRs, so that test_algorithms wont fail anymore.

[senneberden]

You deleted assertion assert isinstance(c, Expression), "Generalization queries first input needs to be a constraint" with a commit message saying that we actually give a relation, not an expression. Should we then not replace the assertion with another one, rather than just deleting it?