title: Detect alternating signs
body: <p>Is there a nice and short way to tell whether a python list (or numpy array) contains numbers with alternating signs? In other words:</p>
  
  <pre><code>is_alternating_signs([1, -1, 1, -1, 1]) == True
  is_alternating_signs([-1, 1, -1, 1, -1]) == True
  is_alternating_signs([1, -1, 1, -1, -1]) == False
  </code></pre>