Added option single to Instance.asdict() allowing to saving in multi-entity format

bindings/python/dlite-entity-python.i

@@ -669,14 +669,25 @@ def get_instance(id: str, metaid: str = None, check_storages: bool = True) -> "I
             iterfun(self),
         )
 
-    def asdict(self, soft7=True, uuid=True):
+
+    def asdict(self, soft7=True, uuid=True, single=True):
         """Returns a dict representation of self.
 
         Arguments:
             soft7: Whether to structure metadata as SOFT7.
             uuid: Whether to include UUID in the dict.
+            single: Whether to return in single-entity format.
+                If None, single-entity format is used for metadata and
+                multi-entity format for data instances.
         """
-        d = OrderedDict()
+        dct = d = OrderedDict()
+        if single is None:
+            single = self.is_meta
+
+        if not single:
+            d = {}
+            dct[self.uuid] = d
+
         if uuid:
             d['uuid'] = self.uuid
         if self.uri:
@@ -700,7 +711,7 @@ def get_instance(id: str, metaid: str = None, check_storages: bool = True) -> "I
         if self.has_property('relations') and (
                 self.is_meta or self.meta.has_property('relations')):
             d['relations'] = self['relations'].tolist()
-        return d
+        return dct
 
     def asjson(self, indent=0, single=None,
                urikey=False, with_uuid=False, with_meta=False,