title: In a Pandas categorical, what is format="table"?
body: <p>The HDF5 format apparently does not support categoricals with format="fixed".  The following example</p>
  
  <pre><code>s = pd.Series(['a','b','a','b'],dtype='category')
  s.to_hdf('s.h5','s')
  </code></pre>
  
  <p>Returns the error:</p>
  
  <blockquote>
    <p>NotImplementedError: Cannot store a category dtype in a HDF5 dataset that uses format="fixed". Use format="table".</p>
  </blockquote>
  
  <p>How do I construct the categorical series with format='table'?</p>