gh-36705: add method to compute the length of a tree-decomposition
Given a graph `G` and a tree-decomposition `T` of `G`, the _length_ of
the tree-decomposition `T` is the maximum _diameter_ in `G` of its bags,
where the diameter of a bag `X_i` is the largest distance in `G` between
the vertices in `X_i`.
We add a method to compute the _length_ of a given tree-decomposition of
a graph.
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URL: https://github.com/sagemath/sage/pull/36705
Reported by: David Coudert
Reviewer(s): David Coudert, Kwankyu Lee
