Added eigenvector centrality

[Amritha16]

[Valliammai19]

Eigenvector centrality is a useful centrality measure. Although Boost has its directed variant, Pagerank @Amritha16 and I couldn't find an implementation of Eigenvector centrality

[jeremy-murphy]

Thanks for contributing this idea. The two algorithms look almost identical. Rather than duplicate the code with a minor change, I think it would be much better to rename the page_rank code to a general name, node_rank or whatever, parameterize it so that then page_rank and eigenvector_centrality just use node_rank with a functor that calculates that u_rank_out. How does that sound?

Some unit tests wouldn't go astray either. ;)

[Amritha16]

@jeremy-murphy That sounds much better than our initial commit. Thanks for the suggestion!:) @Valliammai19 and I will get on this next!