[bugfix] Use method 'exists', instead of 'entry', as originally intended

I mixed up the names when writing the code.  It was always meant to be 'exists'.
No idea how that could ever work.

dune/tnnmg/functionals/sumfunctional.hh

@@ -3,6 +3,7 @@
 
 #include <memory>
 
+#include <dune/common/shared_ptr.hh>
 #include "dune/tnnmg/functionals/nonsmoothconvexfunctional.hh"
 
 namespace Dune {

dune/tnnmg/functionals/test/CMakeLists.txt

 set(TESTS
   normfunctionaltest
+  sumfunctionaltest
 )
 
 add_directory_test_target(_test_target)

dune/tnnmg/functionals/test/functionaltest.hh

@@ -213,7 +213,7 @@ void testHessian(Functional& functional,
               neighborValues[ii] = functional(neighborPoints[ii]);
 
             // The current partial derivative
-            double derivative = hessian.entry(i,j) ? hessian[i][j][k][l] : 0.0;
+            double derivative = hessian.exists(i,j) ? hessian[i][j][k][l] : 0.0;
 
             double finiteDiff = (neighborValues[0]
                   - neighborValues[1] - neighborValues[2]