Add Graph difference

[nahumsa]

Related to #440

Adds the difference between two graphs for PyDiGraph and PyGraph.

[coveralls]

Pull Request Test Coverage Report for Build 2779975375

• 62 of 62 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.001%) to 97.131%

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Totals
Change from base Build 2778191284:	-0.001%
Covered Lines:	12425
Relevant Lines:	12792

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💛 - Coveralls

[georgios-ts]

Users have only indirect control over node indices since we assign them internally. In union we distinguish two nodes/edges based on their data payloads and it's probably better to do the same in this PR too. In more detail I'm thinking that we should:

1. Verify that we can match every node of the first graph into a unique node in the second graph with the same data payload.
2. Remove an edge e = (u, v) from the first graph if the second graph has an edge with endpoints e' = (m[u], m[v]), where m is the matching found in step (1.), and the edge data payloads agree.