title: standard deviation as matlab performance function
body: <p>I Would like to set neural network performance function to standard deviation using net.performfcn.
  Using <code>help(net.performFcn)</code>, seems there are these possibilities:</p>
  
  <pre><code>net.performfcn = 'mse'
  net.performfcn = 'msereg'
  net.performfcn = 'mae'
  net.performfcn = 'sse'
  net.performfcn = 'sae'
  </code></pre>
  
  <p>no standard deviation in this list!
  do I have to implement it myself or there is a quick way that I am not aware of?</p>