object selection fix

system/dataset.py

@@ -68,6 +68,8 @@ class Dataset:
     def extract_id_from_index(self,dim_name,index):
         """Returns a set of ids from array/list of indexes."""
         dim_ids = self.ids[dim_name]
+        if type(index)==int:
+            index = [index]
         return set([id for id,ind in dim_ids.items() if ind in index])
 
     def extract_index_from_id(self,dim_name,id):