Faster uid update function

[nwlandry]

This PR implements two things:

• A more efficient function to update the _edge_uid attribute.
• An improved copy() method (simply copies the _edge_uid counter)

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[maximelucas]

Looks good thank Nich. Any idea how much faster it is?

[nwlandry]

Looks good thank Nich. Any idea how much faster it is?

In the add_edges_from function via format5, the methods are almost the same speed, but for add_edge or add_edges_from with format2 or format4, the new way is a lot faster.