Fix compilation

ATM, only works for Array<T>.

src/utils/Messenger.hpp

@@ -125,7 +125,7 @@ class Messenger
                   &(gather_array[0]), data_array.size(),  mpi_datatype,
                   MPI_COMM_WORLD);
 #else // PASTIS_HAS_MPI
-    static_assert(false, "NIY");
+    gather_array = copy(data_array);
 #endif // PASTIS_HAS_MPI
   }