title: Multiply scipy.lti transfer functions
body: <p>I have the following scipy.lti object that is basically an object representing a Laplace transform of an LTI system:</p>
  
  <pre><code>G_s = lti([1], [1, 2])
  </code></pre>
  
  <p>How to multiply such a transfer function with another one, such as i.e.:</p>
  
  <pre><code>H_s = lti([2], [1, 2])
  
  #I_s = G_s * H_s &lt;---- How to multiply this properly?
  </code></pre>
  
  <p>I guess I could do </p>
  
  <pre><code>I_s = lti(np.polymul([1], [2]), np.polymul([1, 2], [1, 2]))
  </code></pre>
  
  <p>But what if I want to do:</p>
  
  <pre><code>#I_s = H_s / (1 + H_s) &lt;---- Does not work since H_s is an lti object
  </code></pre>
  
  <p>Is there an easy way to do this with scipy?</p>