Issue 573: Warn if target or source on an ObservableRelationship are not Observables

[ajnelson-nist]

This Pull Request resolves all backwards-compatible requirements of Issue #573 .

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 against develop
• [x] CI passes in UCO current unstable branch (c30810b)
• [x] CI passes in CASE current unstable branch tracking UCO's unstable as submodule (aa76a50)
• [x] Impact on SHACL validation reviewed for CASE-Corpora
• [x] Impact on SHACL validation remediated for CASE-Corpora (N/A)
• [x] Impact on SHACL validation reviewed for CASE-Examples
• [x] Impact on SHACL validation remediated for CASE-Examples
• [x] Impact on SHACL validation reviewed for casework.github.io
• [x] Impact on SHACL validation remediated 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]

CASE-Examples and the CASE website both have a few ObservableRelationship objects that could be impacted by the results of the "Just one Observable, or both Observables?" discussion on Issue 573.

[ajnelson-nist]

This PR is ready for review and merge.