volume(G, S, weight=None)¶
Returns the volume of a set of nodes.
The volume of a set S is the sum of the (out-)degrees of nodes in S (taking into account parallel edges in multigraphs). 
- G (NetworkX graph)
- S (sequence) – A sequence of nodes in
- weight (object) – Edge attribute key to use as weight. If not specified, edges have weight one.
The volume of the set of nodes represented by
Sin the graph
 David Gleich. Hierarchical Directed Spectral Graph Partitioning. <https://www.cs.purdue.edu/homes/dgleich/publications/Gleich%202005%20-%20hierarchical%20directed%20spectral.pdf>