Add links to research papers in the section explaning harmonic centrality

[sebastian-nagel]

Eventually add links to

• the Boldi / Vigna paper

  @Article{BoldiVigna:2013:AxiomsCentrality,
    author       = "Paolo Boldi and Sebastiano Vigna",
    title        = "Axioms for Centrality",
    journal      = "CoRR",
    volume       = "abs/1308.2140",
    year         = "2013",
    URL          = "http://arxiv.org/abs/1308.2140",
    pdf          = "https://arxiv.org/pdf/1308.2140.pdf",
    cc-keywords  = "web graph, ranking, harmonic-centrality",
  }
  

• Boldi's talk

  @Misc{Boldi:2013:centrality-measures,
    author       = "Boldi, Paolo",
    year         = "2013",
    title        = "A modern view of centrality measures",
    URL          = "https://www.youtube.com/watch?v=cnGJtGP4gL4",
  }
  

• the link to Wikipedia could point directly to the section: https://en.wikipedia.org/wiki/Centrality#Harmonic_centrality