[yaspgrid] Silence lots of compiler warnings in clang

The YaspGrid entity triggers an insane amount of compilation warnings when compiled with clang (see
the CI logs for examples).

This patch fixes the problem by adding the additional braces requested by the compiler.

dune/grid/yaspgrid/yaspgridentity.hh

@@ -71,12 +71,12 @@ namespace Dune {
       static constexpr std::array<int, sizeof...(I)> computeValues(std::index_sequence<I...>)
         {
           int r = 0, c = 0;
-          return { ((void)I, nextValue(r, c))... };
+          return {{ ((void)I, nextValue(r, c))... }};
         }
 
       template<std::size_t... I>
       static constexpr std::array<int, sizeof...(I)> computeOffsets(std::index_sequence<I...>)
-        { return { (I*(I+1)/2)... }; }
+        { return {{ (I*(I+1)/2)... }}; }
 
       static constexpr std::array<int,(n+1)*(n+2)/2> _values = computeValues(std::make_index_sequence<(n+1)*(n+2)/2>{});
       static constexpr std::array<int,n+1> _offsets = computeOffsets(std::make_index_sequence<n+1>{});
@@ -134,7 +134,7 @@ namespace Dune {
       static constexpr std::array<int, sizeof...(I)> computeOffsets(std::index_sequence<I...>)
         {
           int offset = 0;
-          return { (nextOffset(offset, I))... };
+          return {{ (nextOffset(offset, I))... }};
         }
 
       // compute shift table entry for (`codim`, `i`) and advance `codim`, `i`
@@ -155,7 +155,7 @@ namespace Dune {
       static constexpr std::array<unsigned char, sizeof...(I)> computeValues(std::index_sequence<I...>)
         {
           int codim = 0, i = 0;
-          return { ((void)I, nextValue(codim, i))... };
+          return {{ ((void)I, nextValue(codim, i))... }};
         }
 
       static constexpr std::array<int,dim+1> _offsets = computeOffsets(std::make_index_sequence<dim+1>{});