title: Python Pandas: Create cumulative average while grouping by other column
body: <p>Imagine a table like this:</p>
  
  <pre><code>name | value 
  -----|------
  Jack | 0    
  Jack | 1
  Jack | 0.5
  Jack | 1
  Jill | 0
  Jill | 2
  </code></pre>
  
  <p>For every name, I'd like to have the cumulative average, like this:</p>
  
  <pre><code>name | value | cumAverage
  -----|-------|-----------
  Jack | 0     | 0
  Jack | 1     | 0.5
  Jack | 0.5   | 0.5
  Jack | 1     | 0.625
  Jill | 0     | 0
  Jill | 2     | 1
  </code></pre>
  
  <p>So whenever a new name appears, the cumulative average should "restart".
  The name column is sorted, so whenever a new name appears the current cumulative average is finished.</p>