Function k_shell 

pub fn k_shell(graph: &Graph, k: Option<usize>) -> KCoreResult

Returns the k-shell of the graph.

The k-shell is the subgraph induced by nodes with core number exactly k. That is, nodes in the k-core that are not in the (k+1)-core. If k is None, returns the outer shell (nodes with maximum core number).

Matches networkx.algorithms.core.k_shell.