[numerics] doxygen, add return type to NM_convert
[numerics] doxygen, add return type to NM_convert

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