title: Equivalent of pandas.Series.unique() for non-hashable elements
body: <p>I would like to know if there is an equivalent for pandas.Series.unique() when the series contains non-hashable elements (in my case, lists).</p>
  
  <p>For instance, with </p>
  
  <pre class="lang-py prettyprint-override"><code>&gt;&gt; ds
                     XTR
  s0b0_VARC-0.200    [0.05, 0.05]
  s0b0_VARC-0.100    [0.05, 0.05]
  s0b0_VARC0.000     [0.05, 0.05]
  s0b0_VARC0.100     [0.05, 0.05]
  s0b1_VARC-0.200    [0.05, 0.05]
  s0b1_VARC0.000     [0.05, 0.05]
  s0b1_VARC0.100     [0.05, 0.05]
  s0b2_VARC-0.200    [0.05, 0.05]
  s0b2_VARC-0.100    [0.06, 0.025]
  s0b2_VARC0.000     [0.05, 0.05]
  s0b2_VARC0.100     [0.05, 0.05]
  </code></pre>
  
  <p>I would like to get</p>
  
  <pre class="lang-py prettyprint-override"><code>&gt;&gt; ds.unique()
  2
  </code></pre>