title: Split 2D numpy array with condition
body: <p>I want to split a 2D array based on the value in row 13.
  Input is a 2D Numpy array, but I cant figure out how to create Arrays as an output, as np.append gives me tupels.
  (So I can for example not use np.median on it)</p>
  
  <pre><code>import numpy as np
  
  D = np.load('train_data.npy')
  
  E = []
  F = []
  
  for i, item in enumerate(D):
      if D[i][13]==0:
          np.append(E,item)
      else:
          np.append(F,item)
  
  threshold=np.median(E[:, 7])
  #Throws error, because E is a now tuple. It works on D.
  </code></pre>