Add node centrality metrics to experiment samples

[castarco]

In order to see how the "centrality" of a node could affect to its vision on the blockchain, we could compute those metrics and register them in the experiment logs.

It's important to note that there's not a unique way to measure centrality, and that most of them are designed/thought for undirected graphs, although they are usually easy to adapt. Some potentially useful metrics are (although we should first check which ones make more sense to compute):

• In-Degree Centrality
• Betweenness Centrality
• Closeness Centrality
• Harmonic Centrality
• Eigenvector Centrality