Fix read binary for scalars. Props to Patrick Jaap for pointing it out.

dune/matrix-vector/genericvectortools.hh

@@ -75,8 +75,7 @@ struct ScalarSwitch<Scalar, EnableNumber<Scalar>> {
   }
 
   static void readBinary(std::istream& s, Scalar& v) {
-    for(auto&& vi: v)
-      s.read(reinterpret_cast<char*>(&v), sizeof(Scalar));
+    s.read(reinterpret_cast<char*>(&v), sizeof(Scalar));
   }
 
   template <class BitVector>