title: HDFStore updating stored HDF5 python pandas dataframe
body: <p>I have two dataframes, <code>df1</code> which is stored in a <code>pd.HDFStore</code> object and the other which is to be appended to the dataframe. </p>
  
  <pre><code>store = pd.HDFStore('dataframe_store.h5')
  
  df1 = pd.DataFrame(np.empty((100, 5)))
  df2 = pd.DataFrame(np.empty((100, 5)))
  
  store['df1'] = df1
  </code></pre>
  
  <p>Effectively, I would like the end result to be equal to ...</p>
  
  <pre><code>store['df1'] = df1.append(df2)
  </code></pre>
  
  <p>I would like to append <code>df2</code> to the stored <code>df1</code>, rather completely overwrite the <code>HDFStore</code> object with a new dataframe.  Is this possible?</p>
  
  <p>Furthermore, when I run the following code I return <code>ValueError can only append to Tables</code>... why is this?</p>
  
  <pre><code>df = pd.DataFrame(np.empty((1000, 5)))
  df2 = pd.DataFrame(np.empty((1000, 5)))
  
  store = pd.HDFStore('store.h5')
  
  store['df'] = df
  
  store.append('df', df2)
  </code></pre>