title: save numpy array in append mode
body: <p>Is it possible to save a numpy array appending it to an already existing npy-file --- something like <code>np.save(filename,arr,mode='a')</code>?</p>
  
  <p>I have several functions that have to iterate over the rows of a large array. I cannot create the array at once because of memory constrains. To avoid to create the rows over and over again, I wanted to create each row once and save it to file appending it to the previous row in the file. Later I could load the npy-file in mmap_mode, accessing the slices when needed.</p>