Method is_divisible

perf/perf_tbb_merge.cpp:29–31

    Iterator out;               // where to put merged sequence
    bool empty()   const {return (end1-begin1)+(end2-begin2)==0;}
    bool is_divisible() const {
        return (std::min)( end1-begin1, end2-begin2 ) > grainsize;
    }
    ParallelMergeRange( ParallelMergeRange& r, split ) {
        if( r.end1-r.begin1 < r.end2-r.begin2 ) {
            std::swap(r.begin1,r.begin2);