title: Counting occurrence of a gender in a dataframe by age
body: <p>I have a dataframe like</p>
  
  <pre><code>age gender
  69  M
  29  F
  61  F
  66  F
  52  M
  </code></pre>
  
  <p>What I would like to know is how many males and females there are in each age group.  Is there somehow I could use <code>groupby</code> to group the data by age and then use <code>agg</code> to count the instances of male and female?</p>