[release] consistently use std::multiplies in CollectiveCommunication::prod

Somehow, the vector version of CollectiveCommunication< MPI_Comm >::prod
passed std::plus to allreduce (instead of std::multiplies). This patch
corrects this flaw.

dune/common/parallel/mpicollectivecommunication.hh

@@ -199,7 +199,7 @@ namespace Dune
     template<typename T>
     int prod (T* inout, int len) const
     {
-      return allreduce<std::plus<T> >(inout,len);
+      return allreduce<std::multiplies<T> >(inout,len);
     }
 
     //! @copydoc CollectiveCommunication::min