[Operators] Fix #43 - Improved exploit max length for dangling operator

[falcaopetri]

This PR implements the modification proposed in #43. I've updated all mining assistants that I could find in master.

This PR would also benefit assistants on other branches (such as the Injective assistant).

I've run some basic tests with -maxad 3 and -maxad 4, comparing mined rules before and after this patch. TSVRuleDiff reports no difference between mined rules.

I can see a performance improvement (though I did not benchmark it).
AMIE queue calls to the last generation drops by about half.

[falcaopetri]

The new commit:

• Fixes the problem pointed in Improve "exploit max length" for Dangling Operator #43 about wrongly filtering out rules from the dangling operator. It was not a bug.
• Improves dangling operator max length exploitation for the case of 0 open variables.