Find the k-cores of a graph.
The k-core is found by recursively pruning nodes with degrees less than k.
See the following reference for details:
An O(m) Algorithm for Cores Decomposition of Networks Vladimir Batagelj and Matjaz Zaversnik, 2003. http://arxiv.org/abs/cs.DS/0310049
||Return the core number for each vertex.|
||Return the k-core of G.|
||Return the k-shell of G.|
||Return the k-crust of G.|
||Return the k-corona of G.|