title: remove zeros from nparray efficiently
body: <p>In the moment I am using this method:</p>
  
  <pre><code>data = np.array([[0, 0, 0, 0, 1, 2, 3, 4, 5, 0, 6, 0, 0], [0, 0, 0, 0, 1, 2,3, 4, 5, 0, 6, 0, 0]])
  index = 0
  idx = []
  for img in range(len(data)):
      img_raw = np.any(data[img])
      if img_raw == 0.0:
          idx.append(index)
      index+=1
  data = np.delete(data, idx, axis=0)
  </code></pre>
  
  <p>Does somebody know a better method?</p>