Make edge-IDs from vert-IDs in build_graph helper
Add more tests for Graph.eulerize
Add Graph.combine (and for Verts, Edges)

Non-conflicting extras will simply be added for both verts and edges.
Conflicting extras will raise exceptions.
Use labels for edge-IDs in tgf

Adds numbers on duplicate labels.
Command line changes for graph combining
Add command-line interface test