ThOR & ParticleFilter: Provide user-friendly error in case predicate is not wrapped in FILTER

When the ParticleFilter is invoked as such:

ParticleFilter( detached_particles, Cut=F.PID_K < 5 )

the application segfaults (spectacularly), as the cut should have been wrapped in a F.FILTER. Ideally, this trivial mistake is caught early on and a friendly error message is provided.

Maybe to get started:

Looking in the Python code, I wonder whether we can test the type of the "Cut" argument already in Python (in the ParticleFilter in agorithms_thor.py). This type seems to be a ComposedBoundFunctor in the incorrect case, and a BoundFunctor in the correct case. However, I'm not sure whether it's intended that a ComposedBoundFunctor (which, by inheritance, is a BoundFunctor) would fail logically and which other types are expected to fail. Input from those familiar with the logic design of the filters would be much appreciated here.