Add public group getter for java and scala (#4838)

* Add public group getter for java and scala

* Remove unnecessary param from javadoc

* Fix typo

* Fix another typo

* Add semicolon

* Fix javadoc return statement

* Fix missing return statement

* Add a unit test

jvm-packages/xgboost4j/src/main/scala/ml/dmlc/xgboost4j/scala/DMatrix.scala

@@ -149,6 +149,14 @@ class DMatrix private[scala](private[scala] val jDMatrix: JDMatrix) {
     jDMatrix.setGroup(group)
   }
 
+  /**
+   * Get group sizes of DMatrix (used for ranking)
+   */
+  @throws(classOf[XGBoostError])
+  def getGroup(): Array[Int] = {
+    jDMatrix.getGroup()
+  }
+
   /**
    * get label values
    *