Isolated start-up and shut-down graphs.

[hjoliver]

Close #4912
Supersede #5036
(Really addresses the original #4903 but specifically just for start-up and shut-down graphs)

Implements isolated initial and final graphs as special non-advancing cycle points, to avoid the pain of eternal dependence on initial and final tasks.

The initial cycle point is called alpha and the final one omega.

• first and last letters of the greek alphabet
• avoids potential confusion with the initial/final/start/stop cycle points of the main graph
• (these names are a minor detail, subject to discussion!)

How it works:

• at start-up, the alpha graph loads and runs to completion
• then the main (cycling) graph loads, and runs to completion
• then the omega graph loads and runs to completion

Also:

• restart works as normal, in any of the graphs
• you can manually trigger tasks in any section, in which case (e.g.) alpha/foo will coexist in the task pool with 3/bar etc.
  • if you need to retrigger an initial install task (e.g.) part way through a workflow run, you may or may not want to pause the main flow while it runs - to handle this we should implement flow pause (which is intended anyway, but not done yet)

EXAMPLE

[scheduling]
    [[graph]]
        alpha = "a => b"
        omega = "x => y"
        R1 = "foo => bar"
[runtime]
    [[a, b, x, y, foo, bar]]

Result:

$ cylc cat-log junk | grep '=> running'
INFO - [alpha/a submitted job:01 flows:1] => running
INFO - [alpha/b submitted job:01 flows:1] => running
INFO - [1/foo submitted job:01 flows:2] => running
INFO - [1/bar submitted job:01 flows:2] => running
INFO - [omega/x submitted job:01 flows:3] => running
INFO - [omega/y submitted job:01 flows:3] => running

Check List

• I have read CONTRIBUTING.md and added my name as a Code Contributor.
• Contains logically grouped changes (else tidy your branch by rebase).
• Does not contain off-topic changes (use other PRs for other changes).
• Applied any dependency changes to both setup.cfg and conda-environment.yml.
• Tests are included (or explain why tests are not needed).
• CHANGES.md entry included if this is a change that can affect users
• Cylc-Doc pull request opened if required at Document alpha and omega graphs. cylc-doc#559
• If this is a bug fix, PRs raised to both master and the relevant maintenance branch.

[hjoliver]

I'm tentatively putting this one back to 8.1.0.

• It's ready to go and shouldn't be hard to review (and the underlying principles were discussed at length and agreed)
• It's a nice feature that many users will like (it makes special behaviour at start much easier to understand and configure)
• It has some nasty conflict potential if left too long

[hjoliver]

Merged master and deconflicted.

[hjoliver]

@dpmatthews - pinging you for a big-picture review (given your involvement in the discussions that led to this).

[hjoliver]

Squashed and rebased.

[hjoliver]

(Rebased)

[hjoliver]

(Rebased)

[hjoliver]

(Rebased)