boxmodels: make them compile with -pedantic

tested on GCC 4.3 and GCC 4.6. Due to an incompatibility between those
two compilers (GCC 4.3 expects that there's no semicolon behind
static_assert(), GCC 4.6 requires one), it only compiles on GCC 4.6.

examples/tutorialproblem_coupled.hh

@@ -130,6 +130,7 @@ public:
     TutorialProblemCoupled(TimeManager &timeManager,
                            const GridView &gridView)
         : ParentType(timeManager, gridView)
+        , eps_(3e-6)
     {
     }
 
@@ -234,7 +235,7 @@ public:
 
 private:
     // small epsilon value
-    static constexpr Scalar eps_ = 3e-6;
+    Scalar eps_;
 };
 }