Issue 543: Designate ObservablePattern a subclass of Pattern, adding namespace import

[ajnelson-nist]

This Pull Request resolves all requirements of Issue #543 .

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 (65cc8a8)
• [x] CI passes in CASE current unstable-2.0.0 branch tracking UCO's unstable as submodule (6e89872)
• [x] Impact on SHACL validation reviewed for CASE-Examples
• [x] Impact on SHACL validation remediated for CASE-Examples (N/A)
• [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 casework.github.io
• [x] Impact on SHACL validation remediated for casework.github.io (N/A)
• [x] Milestone linked
• [x] Solutions Approval vote logged on corresponding Issue (N/A) - bugfix proposal

[ajnelson-nist]

Any member of @ucoProject/maintainers-uco may feel free to merge this PR. I believe it is bugfix-grade, and if you agree, please merge.