title: How to use Dask Pivot_table?
body: <p>I'm Trying to use Pivot_table on Dask with the following dataframe:</p>
  
  <pre><code>    date    store_nbr   item_nbr    unit_sales  year    month
  0   2013-01-01  25       103665      7.0        2013      1
  1   2013-01-01  25       105574      1.0        2013      1
  2   2013-01-01  25       105575      2.0        2013      1
  3   2013-01-01  25       108079      1.0        2013      1
  4   2013-01-01  25       108701      1.0        2013      1
  </code></pre>
  
  <p>When I try to pivot_table as follows:</p>
  
  <pre><code>ddf.pivot_table(values='unit_sales', index={'store_nbr','item_nbr'}, 
                                    columns={'year','month'}, aggfunc={'mean','sum'})
  </code></pre>
  
  <p>I got this error: </p>
  
  <pre><code>ValueError: 'index' must be the name of an existing column
  </code></pre>
  
  <p>And If I just use only one value on index and columns parameters as follows:</p>
  
  <pre><code>df.pivot_table(values='unit_sales', index='store_nbr', 
                                    columns='year', aggfunc={'sum'})
  </code></pre>
  
  <p>I got this error: </p>
  
  <pre><code>ValueError: 'columns' must be category dtype
  </code></pre>