[numerics] use MPI_COMM_WORLD if the communicator is not initialized
[numerics] use MPI_COMM_WORLD if the communicator is not initialized

siconos/siconos,radarsat1/siconos,siconos/siconos,radarsat1/siconos,siconos/siconos,radarsat1/siconos,radarsat1/siconos,siconos/siconos,radarsat1/siconos