Use proper naming of investigation IDs

dm.py

@@ -42,7 +42,7 @@ class Object(EntityContainer):
         super(Object, self).__init__(root)
         self.id = self.entities['baseId']
         self.uuid = self.entities['digitalObjectIdentifier']
-        self.investigation = root.find('entities/entity/investigation/investigationId').text
+        self.investigation_id = root.find('entities/entity/investigation/investigationId').text
 
         node = root.find('entities/entity/uploader/userId')
         self.uploader = node.text if node is not None else None