Make pruning optional for debugging

[v4hn]

We recently reworked pruning (#221, #250, #252). It works more reliable now and can reduce computational overhead quite a lot.
However, for debugging, it can be useful to attempt all possible computations.
This can help to identify culprits and identify longest partial solutions in the current design.

I would propose to add a pruning property to the Task container and allow users to call setPruning(false) if they want to do so.

[rhaschke]

Maybe, instead of defining this property in the top-level task container only, you want to do so at the individual container level?
I also suggest automatically inheriting this property from the parent.

[v4hn]

Maybe, instead of defining this property in the top-level task container only, you want to do so at the individual container level?
I also suggest automatically inheriting this property from the parent.

Yes. I don't think we could implement it easily any other way, because the individual stages need to be able to see the flag.
I think there are also unresolved issues with default sources for inheritance though - I seem to recall we don't support them at the moment.

[rhaschke]

Fixed via #590