Validate and list command: list unused namespaces.

[hjoliver]

Close #7392

Validation, and cylc list option to print all unused (i.e., not in the graph) runtime namespaces.

Example: task f is implicit, tasks e, k and FAM_2 are not used by the graph. (FAM_2 is inherited only by k which is not used):

[scheduling]
    [[graph]]
        R1 = "a & FAM_1 & f"
[runtime]
    [[a, e, f]]
    [[FAM_1]]
    [[b, c, d]]
       inherit = FAM_1
    [[FAM_2]]
    [[k]]
       inherit = FAM_2

Validation:

$ cylc val .
INFO - There are runtime namespaces not used by the workflow:
    * e
    * FAM_2
    * k

List command:

$ cylc list --unused-namespaces
FAM_2
e
k

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).
• 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.
• If this is a bug fix, PR should be raised against the relevant ?.?.x branch.

[hjoliver]

Good to go now.