allow strings as Thing.relations-values

This commit adds the type "string" as possible
value for an entry in the `relations`-slot of
`Thing`. This allows to represent pure links
to "related" objects by storing a key-value
pair where the key is the `id` of a related
record and the value is ''.

This allows for a much nicer representation
of extracted records (see issue
<#275>)

src/things/unreleased.yaml

@@ -260,7 +260,10 @@ slots:
     inlined: true
     inlined_as_list: false
     multivalued: true
-    range: Thing
+    range: Any
+    any_of:
+      - range: Thing
+      - range: string
     relational_role: OBJECT
     symmetric: true
     exact_mappings:

src/types/unreleased.yaml

@@ -104,3 +104,9 @@ slots:
     exact_mappings:
       - dcterms:type
 
+
+classes:
+  Any:
+    description: >-
+      Use this type to state that the value of a slot can by any type.
+    class_uri: linkml:Any