This is a port of the 'block' package to use networkx.Graph objects instead of
dicts (supported by enthought.util.graph) to represent dependency graphs. This
code is currently not in use and is intended as a reference for posterity.

The port was straight-forward and opened up an extensive graph library
maintained by others, but we don't use many graph algorithms in the first place
and introducing new package dependencies is painful, so we decided to not use
the port.
