message: use MPI_COMM_WORLD as default communicator for matrix
  Since the constructor of communicator does not call MPI_Comm_duplicate anymore, we can safely use MPI_COMM_WORLD here.
diff: @@ -163,7 +163,7 @@ protected:
  * The default is the invalid MPI_COMM_NULL.
  */
  explicit Matrix(std::shared_ptr<const Executor> exec,
  - mpi::communicator comm = mpi::communicator(MPI_COMM_NULL));
  + mpi::communicator comm = mpi::communicator(MPI_COMM_WORLD));
  /**
  * Asynchronously communicates the values of b that are shared with other
  