[mechanics_io] Support constant-valued BoundaryCondition and FixedBC
[mechanics_io] Support constant-valued BoundaryCondition and FixedBC

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