Return the kcrust of G.
The kcrust is the graph G with the kcore removed.
Parameters :  G : NetworkX graph
k : int, optional
core_number : dictionary, optional


Returns :  G : NetworkX graph

Raises :  NetworkXError :

See also
Notes
This definition of kcrust is different than the definition in [R173]. The kcrust in [R173] is equivalent to the k+1 crust of this algorithm.
Not implemented for graphs with parallel edges or self loops.
For directed graphs the node degree is defined to be the indegree + outdegree.
Graph, node, and edge attributes are copied to the subgraph.
References
[R173]  (1, 2, 3) A model of Internet topology using kshell decomposition Shai Carmi, Shlomo Havlin, Scott Kirkpatrick, Yuval Shavitt, and Eran Shir, PNAS July 3, 2007 vol. 104 no. 27 1115011154 http://www.pnas.org/content/104/27/11150.full 