Improve (distinction between) mx and mxpartial

[Bart Bogaerts]

alltwovaluedextensions can be expensive, in particular if a lot of relations don't appear in the grounding and hence are useless.

IDP2 solved this efficently. I propose we go back to the way it was solved there. I.e.~modelexpand generates list of three-valued models such that...

And: maybe a warning if some "models" are threevalued

And: maybe tell the solver when he can stop i.e. ~solver.nbmodels = nbmodels/nbdontcareatoms

[Broes De Cat]

modelexpand is intended to generate the requested number of two-valued models. For other purposes, a modelexpandpartial method is available, which returns models which can be partial but for which any extension is a model. Maybe the name should change and it should be decided what happens if n models are requested.