[JTC] Cleanup TODO-List from #320

[christophfroehlich]

To keep track of TODOs mentioned with #320:

• [x] try minimizing the amount of includes #943
• [x] 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
• [x] clean the mess of trajectory msg pointers
• [x] 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
• [x] 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
• [x] Convert assign_interface_from_point into member function #945

[christophfroehlich]

@bmagyar regarding

try refactoring these into trajectory_operations.cpp or into trajectory.hpp directly

The methods

• fill_partial_goal
• sort_to_local_joint_order
• validate_trajectory_msg
• add_new_trajectory_msg

all use class variables, like dof_, params_.joints and so on. You suppose to refactor that without relying on class variables but add them as function parameters?