Feature Request: Add function to compute "hierarchy" metric for selected nodes in structuralhole.py

In the file networkx/networkx/algorithms/structuralholes.py, there lacks a function to return the "hierarchy" value of the nodes in the network. Hierarchy is an important metric compensating the "constraint" metric. Its definition can be founded at:

"The Network Structure of Social Capital," 2000 Research in Organizational Behavior http://faculty.chicagobooth.edu/ronald.burt/research/files/NSSC.pdf

Hierarchy is a form of closure in which a minority of contacts, typically one or two, stand above the others for being more the source of closure.