[bugfix] Correctly iterate over children of MixedElement

The way we currently iterate over children of mxied elements
was plain wrong for nested mixed elements. The tests that
revealed this will soon follow.

python/dune/perftool/pdelab/localoperator.py

@@ -334,7 +334,7 @@ def _list_infos(expr, number, visitor):
     elif visitor.measure == "interior_facet":
         restrictions = (Restriction.NEGATIVE, Restriction.POSITIVE)
     for res in restrictions:
-        for ei in range(element.num_sub_elements() + 1):
+        for ei in range(element.value_size()):
             yield PDELabAccumulationInfo(element_index=ei, restriction=res)