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This fixes a bug where equal edge descriptors would be considered
unequal. Additionally, now Vertex and Edges can be ordered (via >, <,
>=, <=).

This fixes #188

This also properly handles self-loops and thus fixes #180.

The new format fully supports all property map types present in
graph-tool and should be much faster and produce smaller files than the
other text-based formats.

This fixes #178.

This fixes #171

This replaces a custom implementation of Ullmann's code.