graph: ghost node detection fix

[oliver-sanders]

Follow-on fix from #2346

This PR fixes the somewhat over-zealous detection of ghost nodes and handles the case that a task is defined but not used.

Detection of ghost-families has been removed as it is doesn't make sense without analysing the suite's triggers individually which is perhaps beyond the scope of cylc graph.

[hjoliver]

Looks fine, but ideally I think we should have ghost families. Wouldn't that just involve checking if all member tasks are ghosts - not just finding one ghost member - or am I missing something?

[oliver-sanders]

am I missing something?

The problem is that not all tasks in a family will necessarily run at a particular cycle point. For example try graphing this suite on master:

[scheduling]
    cycling mode = integer
    initial cycle point = 1
    [[dependencies]]
        [[[R/1/P2]]]
            graph = task_1
        [[[R/2/P2]]]
            graph = task_2
[runtime]
    [[HPC]]
    [[task_1, task_2]]
        inherit = HPC

cylc graph gets its information from cylc.config.get_graph_raw which performs family substitution so by the time the data gets to cylc_xdot it is not possible to tell which tasks a family represents (without performing a second get_graph_raw call?).

[hjoliver]

OK got it.