added classySlice() method to clarify how itemSliceClass is applied.
added classySlice() method to clarify how itemSliceClass is applied.

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