title: Get the row(s) which have the max value in groups using groupby
body: <p>How do I find all rows in a pandas DataFrame which have the max value for <code>count</code> column, after grouping by <code>['Sp','Mt']</code> columns?</p>
  <p><strong>Example 1:</strong> the following DataFrame, which I group by <code>['Sp','Mt']</code>:</p>
  <pre><code>   Sp   Mt Value   count
  0  MM1  S1   a     **3**
  1  MM1  S1   n       2
  2  MM1  S3   cb    **5**
  3  MM2  S3   mk    **8**
  4  MM2  S4   bg    **10**
  5  MM2  S4   dgd     1
  6  MM4  S2   rd      2
  7  MM4  S2   cb      2
  8  MM4  S2   uyi   **7**
  </code></pre>
  <p>Expected output is to get the result rows whose count is max in each group, like this:</p>
  <pre><code>0  MM1  S1   a      **3**
  2  MM1  S3   cb     **5**
  3  MM2  S3   mk     **8**
  4  MM2  S4   bg     **10** 
  8  MM4  S2   uyi    **7**
  </code></pre>
  <p><strong>Example 2:</strong> this DataFrame, which I group by <code>['Sp','Mt']</code>:</p>
  <pre><code>   Sp   Mt   Value  count
  4  MM2  S4   bg     10
  5  MM2  S4   dgd    1
  6  MM4  S2   rd     2
  7  MM4  S2   cb     8
  8  MM4  S2   uyi    8
  </code></pre>
  <p>Expected output is to get all the rows where <code>count</code> equals max in each group like this:</p>
  <pre><code>   Sp   Mt   Value  count
  4  MM2  S4   bg     10
  7  MM4  S2   cb     8
  8  MM4  S2   uyi    8
  </code></pre>