SHACL validation fails when `sh:datatype` is used -- usually!

[ships]

See TODOs in code:
https://github.com/Docmaps-Project/docmaps/blob/main/docmaps-shapes.ttl#L52
https://github.com/Docmaps-Project/docmaps/blob/main/docmaps-shapes.ttl#L46

xsd:date fails in several cases, even though there are ^^ annotations on those fields in the NT files we are validating. Need to investigate why this is and how to fix it. Notice that there are a few cases that do succeed: https://github.com/Docmaps-Project/docmaps/blob/main/docmaps-shapes.ttl#L121 and in general it works if the specification is simply xsd:string.