title: Get unique values from index column in MultiIndex
body: <p>I know that I can get the unique values of a <code>DataFrame</code> by resetting the index but is there a way to avoid this step and get the unique values directly?</p>
  
  <p>Given I have:</p>
  
  <pre><code>        C
   A B     
   0 one  3
   1 one  2
   2 two  1
  </code></pre>
  
  <p>I can do:</p>
  
  <pre><code>df = df.reset_index()
  uniq_b = df.B.unique()
  df = df.set_index(['A','B'])
  </code></pre>
  
  <p>Is there a way built in pandas to do this?</p>