"verify_sets" does not remove conflicting elements

[its-luca]

In the verify_sets(std::vector<set_t>& sets, uint64_t threshold, size_t rounds) functions the outermost loop should be
for (auto& s: sets) {...} instead of or (auto s: sets) {...}. Otherwise the s.erase(...) call inside the for loop will not modify the set_t elements contained in the sets argument, i.e. the conflicting elements are not removed.