Base class for XGraph.
XGraph allows self-loops and multiple edges
with arbitrary (hashable) objects as
nodes and arbitrary objects associated with edges.
Examples
Create an empty graph structure (a "null graph") with no nodes and no edges
>>> from networkx import *
>>> G=XGraph()
You can add nodes in the same way as the simple Graph class
>>> G.add_nodes_from(xrange(100,110))
You can add edges as for simple Graph class, but with optional edge
data/labels/objects.
>>> G.add_edges_from([(1,2,0.776),(1,3,0.535)])
For graph coloring problems, one could use
>>> G.add_edges_from([(1,2,"blue"),(1,3,"red")])