Implement basic API and core algorithsm

[srilman]

Implement the basic API for daft-graph based on the discussion (#1) as well as some core APIs:

• Connected Components: (regular and strong)
• Centrality: pagerank (+ personalized), parallel_personalized_pagerank
• Traversal: bfs, bfs_paths (vertex-set BFS), shortest_paths, all_shortest_paths, all_paths
• Community: label_propagation, power_iteration_clustering
• Motif: find (GraphFrames-style DSL)
• Message passing: aggregate_messages, pregel
• Also: triangle_count, k_core, cycle detection, maximal_independent_set, random_walks, svd_plus_plus, hyper_anf

[PhysicsACE]

Hi @srilman. If this is currently open, could I take this on?