added Stack to direct_SWORD to fix compiler error with test_optimizations
added Stack to direct_SWORD to fix compiler error with test_optimizations

hoangmle/metaSMT,finnhaedicke/metaSMT,agra-uni-bremen/metaSMT,hoangmle/metaSMT,finnhaedicke/metaSMT,nbruns1/metaSMT,nbruns1/metaSMT,nbruns1/metaSMT,agra-uni-bremen/metaSMT,finnhaedicke/metaSMT,agra-uni-bremen/metaSMT,hoangmle/metaSMT