title: Pandas floating point issue with cumsum and division
body: <p>I've ranked the total sales (TOTAL_2012) in descending order and am trying to get the list of groups to have roughly 25% of the sales thus tier 1 group is grossing the most sales/company. </p>
  
  <pre><code>sales['PERCENT_2012'] = sales['TOTAL_2012'] / sales['TOTAL_2012'].sum() 
  sales['CUM_PERCENT_2012'] = sales['PERCENT_2012'].cumsum()
  </code></pre>
  
  <p>The problem is the CUM_PERCENT_2012 is larger than 100% or 1.00 with values such as 1.0000004 so I know it's a floating point issue, is there a way to enforce precision/round to 2 decimal places. </p>