title: simple_cycles produces output in non-deterministic order?
body: <p>I noticed that the output of NetworkX <code>simple_cycles</code> algorithm may not be deterministic, in the sense that same cycles in the same graph may be detected in a different order when an algorithm is run multiple times. Or the same cycle may be detected with nodes in the cycle not being in the same order all the time.</p>
  
  <p>Is this expected behavior of the algorithm? I glanced through the source code and I don't see any randomization elements in the order of execution/etc. </p>
  
  <p>If it's not to be expected, I'm wondering if there is something that messes up my graph in the background.</p>