Issue 430: Add top-level class and requirements on node identifiers

[ajnelson-nist]

This Pull Request will resolve all requirements of Issue #430 .

Coordination

• [x] Pull Request is against correct branch
• [x] Pull Request is in, or reverted to, Draft status before Solutions Approval vote has passed.
• [x] CI passes in UCO feature branch
• [x] CI passes in UCO current unstable branch (98ec576)
• [x] CI passes in CASE current unstable branch tracking UCO's unstable as submodule (db3fe86)
• [x] Impact on SHACL validation reviewed for CASE-Examples
• [x] Impact on SHACL validation remediated for CASE-Examples (Deferred)
• [x] Impact on SHACL validation reviewed for casework.github.io
• [x] Milestone linked
• [x] Solutions Approval vote logged on corresponding Issue (once logged, can be taken out of Draft PR status)

[ajnelson-nist]

Thanks for the approval @eoghanscasey ! I'm going to wait until the versionIRI merges happen before pushing the big green button on this one.