title: Applying Group on Rows in Excell SpreadSheet and get total of them
body: <p>I create an applictaion that creat excel file using OpenXML 
  At one of spreadsheets I want to group rows and get total of values of specific (colomn in row) </p>
  
  <pre>
  **Student           Subject       Score**
  Ahmed                 A          20
  Ahmed                 B           15
  Ahmed                 C           15
  Ahmed Total                      50
  Aly                 A            10
  Aly                 B            20
  Aly                 C            10
  Aly Total                        40
  Grand Total                      90
  </pre>
  
  <p>I need to group scores for each student to get his score 
   then group all scores to get grand total and the style of grouping is the same with style in Ribbon in Excel</p>
  
  <p>I found class GroupItems but I haven't any tutorial for it </p>
  
  <p>How can I do it using OpenXMl + C#</p>