save-csv uses label for node ids

[qiemem]

This results in nodes having no idea on export and thus being impossible to reimport.

[qiemem]

This seems to be fundamental to gephi. I think the only answer may be to remove support for their CSV format. It doesn't appear to affect any other format. Furthermore, CSV network files are super easy to read with the CSV extension. Might put in an example of how to do that in the docs.