Riccati solver with hamiltonian matrices as inputs.
(Ec)   A'*X + X*A + X*R*X -Q = 0.
with the calling sequence [X1,X2,zero]=ric_descr(H), the solution X is given by X=X1/X2.
with the calling sequence [X1,X2,err]=ric_descr(H,E), the solution X is given by X=X1/X2.