ss2tf conversion from state-space to transfer function

## Syntax

                h = ss2tf(sl)
                h = ss2tf(sl,"b")
                h = ss2tf(sl,rmax)
                [Ds, NUM, chi] = ss2tf(..)

## Arguments

sl

  linear system (`syslin` list)

h

  transfer matrix

## Description

Called with three outputs `[Ds,NUM,chi]=ss2tf(sl)` returns the numerator polynomial matrix `NUM`, the characteristic polynomial `chi` and the polynomial part `Ds` separately i.e.:

    h = NUM/chi + Ds

Method:

One uses the characteristic polynomial and `det(A+Eij)=det(A)+C(i,j)` where `C` is the adjugate matrix of `A`.

With `rmax` or `"b"` argument uses a block diagonalization of sl.A matrix and applies "Leverrier" algorithm on blocks. If given, `rmax` controls the conditioning (see bdiag).

## Examples

    s=poly(0,'s');
    h=[1,1/s;1/(s^2+1),s/(s^2-2)]
    sl=tf2ss(h);
    h=clean(ss2tf(sl))
    [Ds,NUM,chi]=ss2tf(sl)

## See also

- tf2ss
- syslin
- nlev
- glever