Fixed: inconsistency check in Setup::AddClause() was incomplete

* The guard incons_[incons.size() - 1] made no sense because
  inconsistency may have been detected at k = 1 but not yet at k = 0.
* When for a grounding of a clause inconsistency has been detected at
  level k, all l > k are marked as inconsistent as well. Only if k = 0
  the inconsistency check is done, otherwise we need to continue
  because a subsequent grounding may prove inconsistency for l < k.
  The former case is now handled by `if (k == 0) return', the latter
  case gets no special treatment because the subsequent iterations test
  for incons_[k].