Support loading COCO detections from a list of predictions

[tadejsv]

This is how this is done in pycocotools: basically, the file is only a list of predictions (what would usually be under the annotations key).

The right API for this would mimic the one in pycocotools: basically, tou would do gts.load_coco_preds(file_name), where the resulting Detections would inherit all attributes from gts, and get the detections from the predictions file.