[functions.affine] default b to 0

dune/stuff/functions/affine.hh

@@ -81,7 +81,8 @@ public:
                                          cfg.get("name", default_cfg.get<std::string>("name")));
   } // ... create(...)
 
-  explicit Affine(const MatrixType& matrix, const RangeType& vector, const std::string name_in = static_id())
+  explicit Affine(const MatrixType& matrix, const RangeType& vector = RangeType(0),
+                  const std::string name_in = static_id())
     : A_(matrix)
     , b_(vector)
     , name_(name_in)