Improve the compatibility of NetworkX

[yecol]

Currently the compatibility of NetworkX API is in preview.
We will make it solid in these aspects:

• graph manipulations
• built-in algorithms for large graphs in the distributed setting

[acezen]

0.4.0 release tasks:

• allow tuple/float/bool node type.
• make project_to_simple implicit and make the session of nx.Graph same with gs.Graph (if transform)
• improve the compatibility of api of Graph and DiGraph, and full pass the Graph/DiGraph tests of NetworkX.
  • implement deepcopy of to_directed and to_undirected in c++;
  • implement GraphView in C++ (including to_directed/to_undirected/reverse).
  • induce subgraph/edgeSubgraph
  • keep a record of the self-loop number in graph.
• improve document of networkx module.
  • tutorial
  • document
• remove the experiment tag
• case study of algorithms.

[yecol]

break-down to detailed issues.