Use correct type for second template argument of `std::array`

Using unsigned long instead of size_t only works on architectures where the two types are identical, but not on, for example, i386. On architectures where the types differ, the generic version would be used instead of the specialization for std::array.

Bug-Debian: https://bugs.debian.org/860666

dune/istl/io.hh

@@ -599,7 +599,7 @@ namespace Dune {
   }
 
   // Recursively print all the blocks -- specialization for std::array
-  template<class K, unsigned long n>
+  template<class K, std::size_t n>
   void writeVectorToMatlabHelper (const std::array<K,n>& v, std::ostream& s)
   {
     for (const auto& entry : v)