Squashed commit of the following:

commit d7948acb
Author: Fabien Maniere <fabien.maniere@cnrs.fr>
Date:   Wed Jan 15 17:36:49 2025 +0100

    export to GEXF: adding the weight attribute to all nodes

commit d937e72e
Author: Fabien Manière <fmaniere.pro@gmail.com>
Date:   Wed Apr 24 17:09:27 2024 +0200

    adding weight attribute