title: Save data with key as npz
body: <p>I have a matrix <code>A</code> and I need to save it as a <code>npz</code> file, labeled with the <code>height</code> key. How can I do that? </p>
  
  <p>To save a random matrix as <code>npz</code> I'm using the following code:</p>
  
  <pre><code>import numpy as np
  Test_matrix = np.random.rand(10,10)
  np.savez('Matrix.npz', Test_matrix)
  </code></pre>
  
  <p>But if I load the file and look for <code>height</code>, nothing is found:</p>
  
  <pre><code>M = np.load('Matrix.npz')
  MM = M['height'].reshape(512,512)
  </code></pre>
  
  <p>Gives the error <code>'height is not a file in the archive'</code></p>