Add methods SwhGraph::{iter_nodes, par_iter_nodes}

Depends on !740

Jenkins job DGRPH/gitlab-builds #2801 failed in 19 min.
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added 5 commits

• 189a9e89...c0af4bda - 2 commits from branch master
• 3a77bad3 - Update to webgraph 0.2.0
• 2c83a658 - Simplify type of deserialized VecGraphs
• 26ff1bc9 - Add methods SwhGraph::{iter_nodes, par_iter_nodes}

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Jenkins job DGRPH/gitlab-builds #2809 failed in 31 min.
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