job kill - set task other outputs

[hjoliver]

Currently cylc kill <task-id> does this:

• kills the associated job
• sets the failed output on the job

This should remain the default, but sometimes we want the scheduler to proceeded differently after the job kill, e.g.:

• kill the job, but carry on as if it succeeded, or as if other real outputs were completed
• kill the job, but expire it (i.e., ~~cut off~~ (don't run) the graph branch from that point)

So, to avoid the current multi-step process, we should implement a "set output" option on the command, e.g. (via @dpmatthews):

• cylc kill --out=expired
• cylc kill --out=required
• cylc kill --out=succeeded

Note: we need to check that the task failed message gets ignored if "expired" has been set.

[hjoliver]

(Setting to 8.x milestone for now, but I'm hoping to get this done soon).