graph traversal utility

[hjoliver]

See discussion at #6221 - we need a way for users to easily see what happens downstream of arbitrary points in the graph, particularly prior to staging a runtime intervention.

static

Plot all paths downstream of chosen task(s) in the graph, e.g.:

$ cylc graph [--from-task=4/model] [--max-edges=10] [--stop-cycle=5]

live (GUI)

Plot all paths downstream of chosen task(s) in the graph, as above, but get the result into the datastore somehow.

• could be done with simulation mode, with a special flow number to avoid impacting a subsequent real flow?

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notes

• to see if a cut branch is just a self-terminating side-branch: follow until it dead-ends (probably a side-branch) or hits the next cycle point (probably not a side branch)
• highlight any off-flow prerequisites