Wrong spin call

[sayali-purdue]

File position_control_underactuated.py has a ROS2 node, 'PositionControllerNode', which uses rclpy library. However, 'main' function in this file is using ROS1 spin,'rospy.spin()' call, at line 127. This means the node, 'PositionControllerNode', will not spin and hence its callbacks such as subscriber callbacks will not execute. The 'main' function should use ROS2 spin.