TableClass: extending table works properly with non-matching column names

modules/base/pymod/table.py

@@ -1758,6 +1758,7 @@ class Table(object):
     
     # check that column types are the same in current and new table
     for name in self.col_names:
+      if name in tab.col_names:
         curr_type = self.col_types[self.GetColIndex(name)]
         new_type = tab.col_types[tab.GetColIndex(name)]
         if curr_type!=new_type: