Refactoring examples.

...mple_controllers/cartesian_impedance_example_controller.h

...a_example_controllers/cartesian_pose_example_controller.h

...ample_controllers/cartesian_velocity_example_controller.h

..._example_controllers/joint_impedance_example_controller.h

...a_example_controllers/joint_position_example_controller.h

...a_example_controllers/joint_velocity_example_controller.h

...ontrollers/src/cartesian_impedance_example_controller.cpp

...ple_controllers/src/cartesian_pose_example_controller.cpp

...controllers/src/cartesian_velocity_example_controller.cpp

...le_controllers/src/joint_impedance_example_controller.cpp

...ple_controllers/src/joint_position_example_controller.cpp

...ple_controllers/src/joint_velocity_example_controller.cpp

franka_example_controllers/src/model_example_controller.cpp