Fix signed/unsigned comparison warnings

tests/mpi_test_Messenger.cpp

@@ -46,7 +46,8 @@ void test_allToAll()
   auto exchanged_array = parallel::allToAll(data_array);
 
   for (size_t i=0; i< data_array.size(); ++i) {
-    REQUIRE(exchanged_array[i] == i);
+    const size_t value = exchanged_array[i];
+    REQUIRE(value == i);
   }
 }