Remove data_type, add optional length

Author: rly

linkml_model/model/schema/array.yaml

@@ -23,7 +23,8 @@ classes:
     attributes:
       name:  # the row label
         key: true  # unique when nested (unlike identifier, which is a global identifier)
-      data_type:
+      length:
+        range: integer
       values:
         range: Any
         multivalued: true