title: Adding data to existing h5py file along new axis using h5py
body: <p>I have some sample code that generates a 3d Numpy array -- I am then saving this data into a h5py file using h5 file. How can I then "append" the second dataset along the 4th dimension? Or, how can I write another 3d dataset along the 4th dimension (or new axis) of an existing <code>.h5</code> file? I have read documentation that I could find, and none of the examples seem to address this. My code is shown below:</p>
  
  <pre><code>import h5py
  import numpy as np
  
  dataset1 = np.random.rand(240,240,250);
  dataset2 = np.random.rand(240,240,250);
  
  with h5py.File('data.h5', 'w') as hf:
      dset = hf.create_dataset('dataset_1', data=dataset1)
  </code></pre>