[JTC] Cleanup TODO-List from #320

To keep track of TODOs mentioned with #320:

• try minimizing the amount of includes #943
• check if this is needed at all #943
  https://github.com/ros-controls/ros2_controllers/blob/4c6d7236245c527f275b1e03a7e5242b658dc782/joint_trajectory_controller/include/joint_trajectory_controller/joint_trajectory_controller.hpp#L53-L61
• clean the mess of trajectory msg pointers
• rename read_state_from_hardware
• try refactoring these into trajectory_operations.cpp or into trajectory.hpp directly https://github.com/ros-controls/ros2_controllers/blob/4c6d7236245c527f275b1e03a7e5242b658dc782/joint_trajectory_controller/include/joint_trajectory_controller/joint_trajectory_controller.hpp#L224-L241
• turn compute_error_for_joint into a proper function & unit test #945
• if "RealtimeSubscriber" existed with an API to check for new message & get message that'd make this very readable https://github.com/ros-controls/ros2_controllers/blob/4c6d7236245c527f275b1e03a7e5242b658dc782/joint_trajectory_controller/src/joint_trajectory_controller.cpp#L164-L165
• Convert assign_interface_from_point into member function #945

Start with joint_trajectory_controller.hpp lines 224-241 and joint_trajectory_controller.cpp lines 164-165, then inspect trajectory_operations.cpp and trajectory.hpp. Determine whether the listed helpers belong in those trajectory files and what RealtimeSubscriber API is available. Done means both remaining unchecked TODOs are resolved and the controller remains consistent.