Use enum for directory item types

I tried using a class hierarchy to express something similar
(File, ExecutableFile, ...). However, jsonschema has limited
(AKA no) support for inheritance when declaring range classes.
Moreover, the implementation of "DirectoryDirectoryItem" felt
awkward.

This solution feel more similar to the POSIX analog ("all same").

src/examples/datalad-dataset.yaml

@@ -21,7 +21,7 @@ graph:
         #      # an enum that defines all possible ways directory
         #      # items could be set up (regular file, symlink,
         #      # executable, etc).
-        item_type: file
+        type: file
         #
   # content for outputs.txt
   - id: MD5E-s4--ba1f2511fc30423bdbb183fe33f3dd0f.txt
@@ -64,16 +64,16 @@ graph:
     items:
       .gitmodules:
         #      content: 144d450caf1e6f93af67973261ac6924fdd3169b
-        item_type: file
+        type: file
       .datalad:
         #      content: d3cafe4592eb5837cfd0fc8f4637afc0dd3e7c14
-        item_type: directory
+        type: directory
       script.py:
         #      content: af926ef0c359556ac1d36d71f7e173d97b893ff2
-        item_type: executable_file
+        type: executable_file
       outputs.txt:
         #      content: MD5E-s4--ba1f2511fc30423bdbb183fe33f3dd0f.txt
-        item_type: file
+        type: file
         #  # annotate availability of the entire tree
         #  available_at:
         #    # map storage to storage-specific identifier

src/linkml/datalad-datasets.yaml

@@ -22,6 +22,15 @@ slots:
       Unique identifier.
     range: string
 
+enums:
+  DirectoryItemType:
+    # TODO add 'meaning' to all these
+    permissible_values:
+      file:
+      executable_file:
+      directory:
+      symlink:
+
 classes:
   TypedThing:
     description: >-
@@ -38,7 +47,8 @@ classes:
     mixins:
       - IdentifiedThing
     description: >-
-      File content
+      File content. More or less an analog of the POSIX filesystem file,
+      which is also name-less content.
   Directory:
     is_a: TypedThing
     mixins:
@@ -55,7 +65,8 @@ classes:
         inlined_as_list: false
   DirectoryItem:
     description: >-
-      An item in a directory
+      An item in a directory. Assigns a name and other properties to some
+      content.
     attributes:
       name:
         description: >-
@@ -64,11 +75,10 @@ classes:
         # within a particular context, not globally, but instead here
         # within a single directory
         identifier: true
-      item_type:
+      type:
         description:
           Identifier of the item type.
-        # TODO must become an annotated enum
-        range: string
+        range: DirectoryItemType
 
 
 #  DataladDataset: