title: Add unique groups to DF for each row including sum from other columns
body: <p>I got a DatFrame looking like this:</p>
  <pre><code>ID     field_1     area_1    field_2       area_2    field_3     area_3    field_4      area_4
  1      scoccer     500       basketball    200       swimming    100       basketball   50
  2      volleyball  100       np.nan        np.nan    np.nan      np.nan    np.nan       np.nan
  3      basketball  1000      football      10        np.nan      np.nan    np.nan       np.nan
  4      swimming    280       swimming      200       basketball  320       np.nan       np.nan
  5      volleyball  110       football      160       volleyball  30        np.nan       np.nan 
  </code></pre>
  <p>The original DataFrame has the same structure but containing columns field_1 up to field_30 as well as area_1 to area_30.</p>
  <p>I'd like to add columns to the DF with horiztonal groups depending on the distinct expressions in 'field_x' with the correspondig area summed up... the added columns should look like this:</p>
  <pre><code>ID   group_1     area_1     group_2     area_2     group_3    area_3
          
  1    scoccer     500        basketball  250        swimming   100
  2    volleyball  100 
  3    basketball  1000       football    10
  4    swimming    480        basketball  320         
  5    volleyball  140        football    160
  </code></pre>
  <p>Is there an easy way to realize this?</p>