[numerics] add extern C when including from C++ (swig)
[numerics] add extern C when including from C++ (swig)

radarsat1/siconos,radarsat1/siconos,siconos/siconos,siconos/siconos-deb,fperignon/siconos,radarsat1/siconos,siconos/siconos,siconos/siconos-deb,siconos/siconos-deb,bremond/siconos,radarsat1/siconos,bremond/siconos,fperignon/siconos,bremond/siconos,fperignon/siconos,bremond/siconos,fperignon/siconos,siconos/siconos,radarsat1/siconos,siconos/siconos-deb,bremond/siconos,fperignon/siconos,siconos/siconos,siconos/siconos-deb,siconos/siconos-deb