title: Pandas: Get highest value from a column for each unique value in another column
body: <p>How to get the highest value in one column for each unique value in another column and return the same dataframe structure back.
  Here is a pandas dataframe example?</p>
  
  <pre><code>       reg.nr   counter      value     ID2  categ             date
  1       37367       421     231385      93      A       20.01.2004
  2       37368       428     235156      93      B       21.01.2004
  3       37369       408     234251      93      C       22.01.2004
  4       37372       403     196292      93      D       23.01.2004
  5       55523       400     247141      139     E       24.01.2004
  6       55575       415     215818      139     F       25.01.2004
  7       55576       402     204404      139     A       26.01.2004
  8       69940       402     62244       175     B       27.01.2004
  9       69941       402     38274       175     C       28.01.2004
  10      69942       404     55171       175     D       29.01.2004
  11      69943       416     55495       175     E       30.01.2004
  12      69944       407     90231       175     F       31.01.2004
  13      69945       411     75382       175     A       01.02.2004
  14      69948       405     119129      175     B       02.02.2004
  </code></pre>
  
  <p>Where i want to return the highest value of column "counter" based on the unique value of column "ID2". After the new pandas dataframe should look like this:</p>
  
  <pre><code>       reg.nr   counter      value     ID2  categ             date
  1       37368       428     235156      93      B       21.01.2004
  2       55575       415     215818      139     F       25.01.2004
  3       69943       416     55495       175     E       30.01.2004
  </code></pre>