title: looking for a 3D version of numpy.linalg.norm
body: <p>I'm looking for a build-in function in python. It should compute the frobenius norm of a 3D array.
  My current approach is:</p>
  
  <pre><code>    np.sqrt(np.sum(np.square(x[:,:,:])))
  </code></pre>
  
  <p>but this is too slow for the size of my arrays.
  Any ideas?</p>
  
  <p>Thanks in advance...</p>