title: How to create a dict/object/mapping in an aggregation field
body: <p>I have the following <code>groupedby</code> object in pandas:</p>
  <pre><code>df_grouped = df.groupby(by=['resolution','media_type', 'asset_type']).file_name
  print (type(df_grouped))
  # &lt;class 'pandas.core.groupby.generic.SeriesGroupBy'&gt;
  </code></pre>
  <p>It looks something like this if I apply a list:</p>
  <pre><code>print (df_grouped.apply(list))
  # resolution  media_type  asset_type
  # HD          Video       Feature       [LG_ALPH02...
  #                         Promo         [Alpha&amp;ch.mov]
  #                         Trailer       [LG_ALP178_2...
  # Name: file_name, dtype: object
  </code></pre>
  <p>However, I would like to be able to create a mapping in the right-most column so that I can keep some of the values grouped together. For example, instead of having a list of <code>file_name</code>s in the right column, to have an object of format <code>{file_name: md5}</code> (note: I have a field called <code>md5</code> in my dataframe). In this way, I could do an iteration such as:</p>
  <pre><code>for keys, value_obj in df_grouped.apply(...).items():
      for filename, md5 in value_obj.items():
          print (filename, '--&gt;', md5)
  </code></pre>
  <p>How could I do this? (come to think of it, it would also be pretty neat to create something like a <code>class</code> or custom object to be able to pass to the <code>apply(...)</code> if possible).</p>