Semantics of Nodes() vs GetNodes()

[rcchen]

Nodes() currently returns a generator function that iterates across all the nodes in a graph, while GetNodes() returns the number of nodes in the graph. The latter is easily confused with the former, especially when one is just starting out with Snap.py. I would like to propose renaming the latter function to NumNodes().

The same issue exists with Edges() and GetEdges()

[roks]

GetNodes() is used in many places, so changing its name is not possible for compatibility reasons.