Allow custom order for edges as well as vertices for incidence matrix

[kcrisman]

See ​this SO question, where the question is how to use incidence_matrix in such a way as to order the vertices and the edges as desired. One can specify vertex order, but not edges.

[dcoudert]

To fill the matrix, we just need a mapping from edges to integers. That's the easy part.

The hard parts are

• to check that the given list of edges is exactly the list of edges of the graph without sorting (Python 3)
• to decide what to do if the input list of edges has labels (raise an error or consider only the vertices)
• possibly to deal with multiple edges / loops. We can decide that the method works only for simple (di)graph

[dcoudert]

This should do the job. Note that edges are considered without labels. It is possible to change that if needed.

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New commits:

​94ec80f

trac #27513: allow to specify the order of edges in incidence_matrix

[git]

Branch pushed to git repo; I updated commit sha1. New commits:

​25da883

trac #27513: allow to give edges with labels

[dcoudert]

You can now give edges with labels. This might ease the use of the method.

[embray]

Moving tickets from the Sage 8.8 milestone that have been actively worked on in the last six months to the next release milestone (optimistically).

[dcoudert]

This tickets is awaiting for review. Thanks.