title: Pandas groupby for zero values
body: <p>I have data like this in a csv file</p>
  
  <pre><code>Symbol  Action  Year
    AAPL     Buy  2001
    AAPL     Buy  2001
     BAC    Sell  2002
     BAC    Sell  2002
  </code></pre>
  
  <p>I am able to read it and groupby like this</p>
  
  <pre><code>df.groupby(['Symbol','Year']).count()
  </code></pre>
  
  <p>I get </p>
  
  <pre><code>             Action
  Symbol Year        
  AAPL   2001       2
  BAC    2002       2
  </code></pre>
  
  <p>I desire this (order does not matter)</p>
  
  <pre><code>             Action
  Symbol Year        
  AAPL   2001       2
  AAPL   2002       0
  BAC    2001       0
  BAC    2002       2
  </code></pre>
  
  <p>I want to know if its possible to count for zero occurances</p>