title: Create new column from specific rows in pandas dataframe
body: <p>I have a csv file where each row represents a property followed by a variable number of subsequent rows that reflect rooms in the property. I want to create a column that, for each property, summates the gross floor area of each room. The unstructured nature of the data is making this difficult to achieve in <code>pandas</code>. Here is an example of the table I have at the moment:</p>
  
  <pre><code>id  ba  store_desc      floor_area
  0   1   Toy Shop        NaN
  1   2   Retail Zone A   29.42
  2   2   Retail Zone B   31.29
  3   1   Grocery Store   NaN
  4   2   Retail Zone A   68.00
  5   2   Outside Garden  83.50
  6   2   Office          7.30
  </code></pre>
  
  <p>Here is the table I am trying to create:</p>
  
  <pre><code>id  ba  store_desc      floor_area   gross_floor_area
  0   1   Toy Shop        NaN          60.71
  3   1   Grocery Store   NaN          158.8
  </code></pre>
  
  <p>Does anybody have any pointers on how to achieve this result? I'm totally lost.</p>
  
  <p>Sam</p>