Make default constructors of DenseVector and FieldVector constexpr

Otherwise FieldVector is not a literal type and
calling its size() method is not a const-expression
although size() itself is declared as constexpr.

dune/common/densevector.hh

@@ -244,7 +244,7 @@ namespace Dune {
 
   protected:
     // construction allowed to derived classes only
-    DenseVector () {}
+    DUNE_CONSTEXPR DenseVector () {}
 
   public:
     //===== type definitions and constants

dune/common/fvector.hh

@@ -105,7 +105,7 @@ namespace Dune {
     typedef typename Base::value_type value_type;
 
     //! Constructor making default-initialized vector
-    FieldVector()
+    DUNE_CONSTEXPR FieldVector()
       : _data{}
     {}