* [jvm-packages] call setGroup for ranking task
* passing groupData through xgBoostConfMap
* fix original comment position
* make groupData param
* remove groupData variable, use xgBoostConfMap directly
* set default groupData value
* add use groupData tests
* reduce rank-demo size
* use TaskContext.getPartitionId() instead of mapPartitionsWithIndex
* add DF use groupData test
* remove unused varable
