[io] add opacity management to contact objects
[io] add opacity management to contact objects

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