made __neg__() protein seqtype check optional added absoluteSlice()
made __neg__() protein seqtype check optional
added absoluteSlice()

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