Return the kshell of G.
The kshell is the subgraph of nodes in the kcore containing nodes of exactly degree k.
Parameters :  G : NetworkX graph
k : int, optional
core_number : dictionary, optional


Returns :  G : NetworkX graph

Raises :  NetworkXError :

See also
core_number, k_corona, 
Notes
This is similar to k_corona but in that case only neighbors in the kcore are considered.
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.