[mechanics io] don't fail when read-only if permanent_interactions is not in file
[mechanics io] don't fail when read-only if permanent_interactions is not in file

siconos/siconos,radarsat1/siconos,bremond/siconos,radarsat1/siconos,radarsat1/siconos,radarsat1/siconos,bremond/siconos,fperignon/siconos,fperignon/siconos,radarsat1/siconos,siconos/siconos,fperignon/siconos,siconos/siconos,bremond/siconos,bremond/siconos,fperignon/siconos,siconos/siconos,bremond/siconos,fperignon/siconos