title: Merging/Concat rows in Pandas, based on some specific conditions
body: <p>I have checked all similar questions, but haven't found a solution to what i look for (or at least i don't get it).</p>
  <p>The following is a part of a big DataFrame.</p>
  <p><code>date     venue   result  gf  ga  team     opponent</code><br />
  <code>2017-08-11  Home     W    4   1  Arsenal   Burnley</code><br />
  <code>2017-08-11  Away     L    1   4  Burnley   Arsenal</code><br />
  <code>2017-08-11  Home     D    2   0  Fulham    Leeds</code><br />
  <code>2019-08-11  Home     D    1   0  Arsenal   Burnley</code></p>
  <p>I would like to make it on one row, if <code>opponent == team</code> on the same date.</p>
  <p>So the goal is to get the following:</p>
  <p><code>date     venue   result  gf  ga  team     opponent   gf_opponent  ga_opponent</code><br />
  <code>2017-08-11  Home     W    4   1  Arsenal   Burnley         1             4</code><br />
  <code>2017-08-11  Home     D    2   0  Fulham    Leeds</code><br />
  <code>2019-08-11  Home     D    1   0  Arsenal   Burnley</code></p>
  <p>I want to do that for the whole DataFrame, where <code>opponent == team</code> on the same date.</p>
  <p>Thank you in advance.</p>