tetrahedron ABCD : circumsphere and insphere

a = (a_x, a_y, a_z)  ...  d = (d_x, d_y, d_z)  are the coordinates of the vertices,
N_abc  is a cross product, thus a vector orthogonal to the plane (ABC).
The coordinates of the centers S_C of the circumsphere and S_I of the insphere,
   just as their respective radii R and r, express themselves with determinants and norms.