title: Split dataframe into Excel files by column value
body: <p>I have a DataFrame like this</p>
  
  <pre><code>&gt;&gt;&gt; df
      id    name    score    subject
      0001   'bob'    100    'math'
      0001   'bob'     67    'science'
      0001   'bob'     63    'bio'
      0002  'jack'     67    'math'
      0002  'jack'     98    'science' 
      0002  'jack'     90    'bio'
      0003  'jack'     60    'math'
      0003  'jack'     78    'science' 
      0003  'rose'     87    'bio'
  </code></pre>
  
  <p>I want to filter every <code>id</code>'s data into a new DataFrame and write to an Excel file based on its id. So, the above <code>df</code> will be filtered into 3 DataFrames whose <code>ids</code> are <code>0001</code>, <code>0002</code> and <code>0003</code>, and all the DataFrames will be written to individual excel files.</p>