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
| core_number(G) | Return the core number for each vertex. |
| k_core(G[, k, core_number]) | Return the k-core of G. |
| k_shell(G[, k, core_number]) | Return the k-shell of G. |
| k_crust(G[, k, core_number]) | Return the k-crust of G. |
| k_corona(G, k[, core_number]) | Return the k-crust of G. |