title: pandas: groupby multiple columns, concatenating one column while adding another
body: <p>If I had the following df:</p>
  
  <pre><code>      amount   name   role    desc
  0        1.0    a      x       f
  1        2.0    a      y       g
  2        3.0    b      y       h
  3        4.0    b      y       j
  4        5.0    c      x       k
  5        6.0    c      x       l
  6        6.0    c      y       p
  </code></pre>
  
  <p>I want to group by the <code>name</code> and <code>role</code> columns, add up the <code>amount</code>, and also do a concatenation of the <code>desc</code> with a <code>,</code> :</p>
  
  <pre><code>      amount   name   role    desc
  0        1.0    a      x       f
  1        2.0    a      y       g
  2        7.0    b      y       h,j
  4        11.0   c      x       k,l
  6        6.0    c      y       p
  </code></pre>
  
  <p>What would be the correct way of approaching this?</p>
  
  <p>Side question: say if the <code>df</code> was being read from a .csv and it had other unrelated columns, how do I do this calculation and then write to a new .csv along with the other columns (same schema as the one read)?</p>