Test whether the NormFunctional::subDiff method works

It does.

dune/tnnmg/functionals/test/normfunctionaltest.cc

@@ -48,6 +48,9 @@ int main(int argc, char** argv) try
   // Test the directional subdifferential at the given test points
   testDirectionalSubdifferential(functional, testPoints);
 
+  // Test the partial subdifferential at the given test points
+  testSubDiff(functional, testPoints);
+
   return 0;
 }
 catch (Exception e)