An assortative graph is one where vertexes of the same type are more likely to
have edges between them. The (discrete) assortativity-coefficient measures how
assortative a graph is based on its mixing matrix. The definition we use is from
Mixing Patterns in Networks by Mark Newman. See the citation 'newman200-mixing' in moab
or the URL 'http://arxiv.org/abs/cond-mat/0209450'.