fixed seq[-1] getitem bug, fixed 0 length bug, added "letter graph" getitem interface.
fixed seq[-1] getitem bug, fixed 0 length bug, added "letter graph" getitem interface.

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