Group trigger proposal

[hjoliver]

Following up on an Element chat discussion, and feedback that users find re-running a sub-graphs easier in Cylc 7. Because (if all affected tasks remain in the task pool) they don't have to use the graph structure to identify the initial task(s) and off-flow prerequisites of the sub-graph, or worry about unwanted downstream activity.

This proposal should make it just as easy (but more general) in Cylc 8.

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 if present).