Issue 573: Require target or source on an ObservableRelationship be Observables

[ajnelson-nist]

This Pull Request resolves all backwards-incompatible 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-2.0.0
• [x] CI passes in UCO current unstable-2.0.0 branch (2f780d8)
• [x] CI passes in CASE current unstable-2.0.0 branch tracking UCO's unstable-2.0.0 as submodule (4ac6538)
• [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)
• [x] PR 601 merged

[ajnelson-nist]

This PR is ready for review and merge.