Add K Nearest Neighbor search into STR-Tree (#75)

* Add K Nearest Neighbor search into STR-Tree and add corresponding test case

Signed-off-by: Jia Yu <jiayu2@asu.edu>

* Add Page Header for new files

* Update Page Header for two new files

Signed-off-by: Jia Yu <jiayu2@asu.edu>

* Change API usage and modify JavaDoc

Signed-off-by: Jia Yu <jiayu2@asu.edu>

* Use a private static method to retrieve KNN items

Signed-off-by: Jia Yu <jiayu2@asu.edu>

* Expose KNN to the TestBuilder

Signed-off-by: Jia Yu <jiayu2@asu.edu>

* Convert KNN output to GeometryCollection

Signed-off-by: Jia Yu <jiayu2@asu.edu>

* Add K as a parameter

Signed-off-by: Jia Yu <jiayu2@asu.edu>