Remove constexpr from filledArray().

Fixes #102, and partly #99.

This makes it compatible with C++11 and thus g++ 4.9.

`filledArray()` effectively wasn't constexpr anyway until C++17, where
`std::array` becomes `constexpr` (for the most part, at least).

dune/common/filledarray.hh

@@ -20,18 +20,15 @@ namespace Dune
 
   //! Return an array filled with the provided value.
   /**
-   * \note This function is `constexpr` only in C++17, or, more precisely,
-   *       when `std::array::begin()` and `std::array::end()` are `constexpr`.
-   *
    * \tparam n     Size of the returned array.
    * \tparam T     Value type of the returned array.  This is usually deduced
    *               from `t`.
    */
   template<std::size_t n, class T>
-  constexpr std::array<T, n> filledArray(const T& t)
+  std::array<T, n> filledArray(const T& t)
   {
     std::array<T, n> arr{};
-    // this is constexpr in c++17, `arr.fill(t)` is not
+    // this would be constexpr in c++17, `arr.fill(t)` is not
     for(auto &el : arr)
       el = t;
     return arr;

dune/common/test/filledarraytest.cc

@@ -25,15 +25,5 @@ int main() {
     status = 1;
   }
 
-#ifdef __cpp_lib_array_constexpr
-  std::cout << "The result of Dune::filledArray() is constexpr" << std::endl;
-  constexpr auto test2 = Dune::filledArray<2>(2);
-  (void)test2;
-#else // !__cpp_lib_array_constexpr
-  std::cout << "Not checking whether Dune::filledArray() is constexpr\n"
-            << "since the library does not declare std::array as constexpr\n"
-            << "(__cpp_lib_array_constexpr is not defined)." << std::endl;
-#endif // !__cpp_lib_array_constexpr
-
   return status;
 }