Draft: Add SubgraphWccs
This wraps webgraph's Sccs and a ContiguousSubgraph to transparently provide access to connected components of a subgraph; and does not expose the renumbering to users
This is a draft because it depends on !794 (merged), !797, and is not tested yet. (EDIT: now it is, just not on a real graph)
Edited by vlorentz