title: CVAT coco annotation json - iscrowd option
body: <p>when outputting coco annotation json from CVAT, it is found that there is an attribute call 'iscrowd', but I cant seem to figure out how it is adjusted or annotated on CVAT to change the value to 1, under the scenario where the object is literally crowded. Can any one shed some lights?</p>
  <pre><code>{
    &quot;annotations&quot;: [
      {
        &quot;bbox&quot;: [
          762.703125,
          176.583984375,
          425.27301025390625,
          413.6938171386719
        ],
        &quot;id&quot;: 1,
        &quot;area&quot;: 175932.81493799202,
        &quot;iscrowd&quot;: 0,
        &quot;segmentation&quot;: [
          [
            762.703125,
            176.583984375,
            1187.9761352539062,
            176.583984375,
            1187.9761352539062,
            590.2778015136719,
            762.703125,
            590.2778015136719
          ]
        ],
        &quot;category_id&quot;: 1,
        &quot;image_id&quot;: 0
      }
    ],
    &quot;licenses&quot;: [
      {
        &quot;name&quot;: &quot;&quot;,
        &quot;id&quot;: 0,
        &quot;url&quot;: &quot;&quot;
      }
    ],
    &quot;info&quot;: {
      &quot;version&quot;: &quot;&quot;,
      &quot;description&quot;: &quot;&quot;,
      &quot;date_created&quot;: &quot;&quot;,
      &quot;contributor&quot;: &quot;&quot;,
      &quot;year&quot;: &quot;&quot;,
      &quot;url&quot;: &quot;&quot;
    },
    &quot;images&quot;: [
      {
        &quot;file_name&quot;: &quot;10.jpg&quot;,
        &quot;id&quot;: 0,
        &quot;license&quot;: 0,
        &quot;flickr_url&quot;: &quot;&quot;,
        &quot;height&quot;: 864,
        &quot;coco_url&quot;: &quot;&quot;,
        &quot;date_captured&quot;: 0,
        &quot;width&quot;: 1536
      }
    ],
    &quot;categories&quot;: [
      {
        &quot;name&quot;: &quot;test&quot;,
        &quot;supercategory&quot;: &quot;&quot;,
        &quot;id&quot;: 1
      }
    ]
  }
  </code></pre>
  <p>On the other hand when adding an attribute to a label (Simple Check Box Attribute), it doesn't seem to be properly outputted in COCO json. Am I doing something Wrong?</p>