title: Pandas - write Multiindex rows with to_csv
body: <p>I am using to_csv to write a Multiindex DataFrame to csv files. The csv file has one column that contains the multiindexes in tuples, like:</p>
  
  <pre><code>('a', 'x')
  ('a', 'y')
  ('a', 'z')
  ('b', 'x')
  ('b', 'y')
  ('b', 'z')
  </code></pre>
  
  <p>However, I want to be able to output the Multiindex to two columns instead of one column of tuples, such as:</p>
  
  <pre><code>a, x
   , y
   , z
  b, x
   , y
   , z
  </code></pre>
  
  <p>It looks like <code>tupleize_cols</code> can achieve this for columns, but there is no such option for the rows. Is there a way to achieve this?</p>