[numerics] add a function for computing eigenvalues of a 3x3 symmetrix matrix
[numerics] add a function for computing eigenvalues of a 3x3 symmetrix matrix

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