box richards test: add missing const qualifier

this makes sure that the test does not abort with the error message
"The problem does not provide a initial() method."

git-svn-id: svn://svn.iws.uni-stuttgart.de/DUMUX/dumux/trunk@5922 2fb0f335-1f38-0410-981e-8018bf24f1b0

test/boxmodels/richards/richardslensproblem.hh

@@ -302,7 +302,7 @@ public:
      */
     using ParentType::initial;
     void initial(PrimaryVariables &values,
-                 const GlobalPosition &pos)
+                 const GlobalPosition &pos) const
     { initial_(values, pos); };
 
     // \}