lqg_ltr LQG with loop transform recovery

## Syntax

    [kf, kc] = lqg_ltr(sl, mu, ro)

## Arguments

sl

  linear system in state-space form (`syslin` list)

mu,ro

  real positive numbers chosen ``small enough''

kf,kc

  controller and observer Kalman gains.

## Description

returns the Kalman gains for:

             x = a*x + b*u + l*w1
    (sl)
             y = c*x + mu*I*w2

             z = h*x

Cost function:

             /+oo
             |
    J    = E(| [z(t)'*z(t) + ro^2*u(t)'*u(t)]dt)
     lqg     |
             / 0

The lqg/ltr approach looks for `L,mu,H,ro` such that: J(lqg) = J(freq) where

            /+oo        *  *           *
    J    =  | tr[S  W  W  S ] + tr[T  T]dw
     freq   |
            /0

and

     S = (I + G*K)^(-1)
     T = G*K*(I+G*K)^(-1)

## See also

- syslin