Change spawn command name.

[hjoliver]

These changes close #3735

After discussion on the Issue and in offline chat, I've gone with cylc set-outputs as the new command name. This should be a more intuitively obvious name for users (it makes the scheduler behave just as if specified outputs had been completed naturally).

Notes:

• I have not changed the internal method names that action this command, just the user-facing stuff
• there were already some functional tests that effectively test the command, but only for the default case ("succeeded" outputs); so I've added a functional test to check that setting of custom outputs works.

Requirements 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).
• Appropriate tests are included (functional; plus default case coverage by existing tests).
• Appropriate change log entry included.
• No documentation update required (will be covered by general Cylc 8 documentation upgrade).
• No dependency changes.

[kinow]

I have not changed the internal method names that action this command, just the user-facing stuff

👍

Without re-creating the virtual environment, cylc spawn gives some Python traceback due to missing module. But after re-creating it, it prints the expected output

(venv) kinow@ranma:~/Development/python/workspace/cylc-flow$ cylc spawn
Error: cylc spawn: unknown utility. Abort.
Type "cylc help all" for a list of utilities.

Also did a grep with IDE, found no occurrences of anything similar to cylc spawn. Great job!

Q: is REG an argument? The usage example doesn't seem to use it. Asking because I'm trying to run a simple test with five 😬

[hjoliver]

Q: is REG an argument? The usage example doesn't seem to use it. Asking because I'm trying to run a simple test with five grimacing

Good spotting!

REG stands for "REGistered workflow name" (i.e. from cylc register foo /path/to/foo/flow.cylc). It should be used, I'll fix the help.

(I'll also put an issue up to swap out "REG" for something less obscure, probably "FLOW", in all command help).

[dwsutherland]

Tested & working on my machine, tests passed, code LGTM 👍