Add Louvain community detection algorithm

[Becheler]

Implement #447

• Multi-level modularity optimization following Blondel et al (2008)
• Supports custom quality functions (other than modularity) with policy-based design (extensions to come to propose alternative quality functions to match gen-louvain)
• Incremental quality tracking
• Lazy rollback
• Vertex shuffling
• Competitive with established implementations (I will work on making the benchmarks reproducible)

But:

• tests are not integrated yet to b2
• documentation is not ready
• example is not ready