Merge branch 'feature/constexpr-unpack-integer-sequence' into 'master'

Allow constant expressions in unpackIntegerSequence

See merge request !996

dune/common/indices.hh

@@ -120,7 +120,7 @@ namespace Dune
    * \returns Result of calling f with unpacked integers.
    */
   template<class F, class I, I... i>
-  decltype(auto) unpackIntegerSequence(F&& f, std::integer_sequence<I, i...> sequence)
+  decltype(auto) constexpr unpackIntegerSequence(F&& f, std::integer_sequence<I, i...> sequence)
   {
     return f(std::integral_constant<I, i>()...);
   }