title: groupby list of lists of indexes
body: <p>I have a list of np. arrays, representing indexes of pandas dataframe.</p>
  
  <p>I need to groupby index to get each group for each array  </p>
  
  <p>let's say, that is the df:</p>
  
  <pre><code>index values
  0     2
  1     3
  2     2
  3     2
  4     4
  5     4
  6     1
  7     4
  8     4
  9     4
  </code></pre>
  
  <p>and that is the list of np.arrays:</p>
  
  <pre><code>[array([0, 1, 2, 3]), array([6, 7, 8])]
  </code></pre>
  
  <p>from this data I expect to get 2 groups without loop opertaions as a single groupby object:</p>
  
  <p>group1:</p>
  
  <pre><code>index values
  0     2
  1     3
  2     2
  3     2
  </code></pre>
  
  <p>group2:</p>
  
  <pre><code>index values
  6     1
  7     4
  8     4
  </code></pre>
  
  <p>I would stress again that finally I need to get a single groupby object.</p>
  
  <p>Thank you!</p>