Group trigger

[hjoliver]

Implement group trigger proposal: https://github.com/cylc/cylc-admin/pull/197

(Not approved yet, but it's a clear winner and a relatively small change, and I wanted to finish coding it up - after initial experiments - while still fresh in the brain).

On current master:

• on triggering one or more tasks: run them all immediately regardless of prerequisites.

On this branch:

• on triggering one or more tasks, satisfy any off-group (aka off-flow) prerequisites

Result: we can trigger a sub-graph "naturally" by specifying all of its member tasks:

• tasks with only off-group prerequisites will trigger immediately
• the subsequent flow will respect in-group prerequisites
• no stalling due to unsatisfied off-group prerequisites

This provides an alternative, easier for some cases, to triggering just the initial task(s) of the sub-graph and setting off-flow prerequisites (if any) manually.

---

example

[scheduler]
    allow implicit tasks = True
[scheduling]
    [[graph]]
        R1 = """
            foo => a => b => c => bar
            off => b
        """
[runtime]
    [[root]]
        pre-script = sleep 4
    [[bar]]
        script = "test $CYLC_TASK_SUBMIT_NUMBER != 1"

Run it and wait for bar to fail and stall the workflow.

$ cylc trigger grp //1/a //1/b //1/c --flow=new
  # 1/a runs immediately in flow 2 - its only prerequisite (foo => a) is off-group 
  # 1/b gets its off-group prerequisite satisfied (off => b) and spawns to wait on in-group (a => b)
  # flow 2 traverses a => b => c, then flows on to merge with and run bar to complete the workflow

---

caveats?

Triggering a future sub-graph, or a past sub-graph with --flow=new, is safe and easy.

But triggering a past sub-graph in the original flow comes with a gotcha: setting off-group prerequisites spawns the target task to wait on other prerequisites, which aren't going to get satisfied when the original flow does not re-traverse the graph.

This isn't technically incorrect. The exact same thing would happen if we attempted to trigger the same past sub-graph the spawn-on-demand "native" way in the same flow: trigger the initial task and set the off-flow prerequisite.

But maybe we can do something to mitigate the problem if users try to do this.

---

interaction with the upcoming cylc remove extension

On this branch now, to re-run a past sub-graph you have to use --flow=new.

After the remove extension we'll be able to erase the flow history and re-run a past sub-graph in the original flow. We may want to add an option to cylc trigger to do that automatically, or even make it the default.

---

interaction with upcoming inactive task matching developments

We can't trigger a group of inactive tasks by family name or glob yet, but that's a general problem, not specific to this branch.

---

Check List

• [x] I have read CONTRIBUTING.md and added my name as a Code Contributor.
• [x] Contains logically grouped changes (else tidy your branch by rebase).
• [x] Does not contain off-topic changes (use other PRs for other changes).
• [x] Applied any dependency changes to both setup.cfg (and conda-environment.yml if present).
• [ ] Tests are included (or explain why tests are not needed).
• [ ] Changelog entry included if this is a change that can affect users
• [ ] Cylc-Doc pull request opened if required at cylc/cylc-doc/pull/XXXX.
• [x] If this is a bug fix, PR should be raised against the relevant ?.?.x branch.