simplify mh column transition operator with mh_choose helper
simplify mh column transition operator with mh_choose helper

probcomp/crosscat,probcomp/crosscat,mit-probabilistic-computing-project/crosscat,probcomp/crosscat,probcomp/crosscat,fivejjs/crosscat,fivejjs/crosscat,probcomp/crosscat,probcomp/crosscat,fivejjs/crosscat,fivejjs/crosscat,mit-probabilistic-computing-project/crosscat,mit-probabilistic-computing-project/crosscat,mit-probabilistic-computing-project/crosscat,probcomp/crosscat,mit-probabilistic-computing-project/crosscat,mit-probabilistic-computing-project/crosscat,fivejjs/crosscat,mit-probabilistic-computing-project/crosscat,fivejjs/crosscat,probcomp/crosscat,fivejjs/crosscat