get rid of interval_id_union functions and associated data structure
get rid of interval_id_union functions and associated data structure

cjlee112/pygr,cjlee112/pygr,RitwikGupta/pygr,ctb/pygr,GenDataPro/pygr,theoryno3/pygr,ctb/pygr,GenDataPro/pygr,GenDataPro/pygr,ctb/pygr,RitwikGupta/pygr,cjlee112/pygr,theoryno3/pygr,RitwikGupta/pygr,theoryno3/pygr,ctb/pygr,ctb/pygr,cjlee112/pygr