Allow top-level block field data to be viewed in the spreadsheet view

There is no current way to view only the field data associated with the top-level block. Steps to reproduce:

1. Open can.ex2
2. Create a Cell Size filter and enable Compute Sum
3. Create a SpreadSheet view and show Field Data attributes

At the top-level block there is an array called Volume with value of 368 but it doesn't appear that there's any way to conveniently see this value.