Implemented Issue 380

[b0bkaT]

Changed observable:inReplyTo to an owl:DatatypeProperty with an rdfs:range of xsd:string.

This Pull Request resolves all requirements of Issue 380.

Review steps taken:

• Pull request is against correct branch
• CI passes in (CASE/UCO) feature branch
• CI passes in (CASE/UCO) current unstable branch (merge-commit)
• Impact on SHACL validation reviewed for CASE-Examples
• Impact on SHACL validation remediated for CASE-Examples (N/A)
• Impact on SHACL validation reviewed for casework.github.io
• Impact on SHACL validation remediated for casework.github.io (N/A)

[ajnelson-nist]

Thanks, @b0bkaT !

I'm converting this to Draft until the Solutions Approval vote passes.