added IDGraph that uses IDs for internal storage, and can work with persistent storages like shelve or SQLGraph.
added IDGraph that uses IDs for internal storage, and can work with persistent
storages like shelve or SQLGraph.

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