title: Using linq.js, I need to group by 2 fields and produce summary
body: <p>I have json as </p>
  
  <pre><code>var obj = [{projectName:"Test1", catName: "subTest1, marks1: 25, marks2: 45},
  {projectName:"Test1", catName: "subTest1", marks1: 45, marks2: 45},
  {projectName:"Test1", catName: "subTest2", marks1: 25, marks2: 45}
  {projectName:"Test1", catName: "subTest2", marks1: 15, marks2: 25},
  {projectName:"Test2", catName: "subTest1", marks1: 35, marks2: 45}
  {projectName:"Test2", catName: "subTest2", marks1: 20, marks2: 55}
  {projectName:"Test2", catName: "subTest2", marks1: 25, marks2: 65}
  
  ]
  </code></pre>
  
  <p>I need to group by projectName and catName and sum each marks achieve a result as below</p>
  
  <pre><code>result = [{projectName:"Test1", catName: "subTest1", marks1: 70, marks2: 90},
  {projectName:"Test1", catName: "subTest2", marks1: 40, marks2: 70}
  .....]
  </code></pre>
  
  <p>Please help.</p>