title: Convert int64 to uint64
body: <p>I want to convert an int64 numpy array to a uint64 numpy array, adding 2**63 to the values in the process so that they are still within the valid range allowed by the arrays. So for example if I start from</p>
  
  <pre><code>a = np.array([-2**63,2**63-1], dtype=np.int64)
  </code></pre>
  
  <p>I want to end up with</p>
  
  <pre><code>np.array([0.,2**64], dtype=np.uint64)
  </code></pre>
  
  <p>Sounds simple at first, but how would you actually do it?</p>