Few toughts on simplifying controllers (JTC, admittance,...) that use position, velocity and acceleration interfaces (and combinations of those)

There is a quite a bunch of same code and logic needed in those controllers.
We could add an abstraction layer to it.
Here is a short proposal on this:

• Adding KinematicsControllerBase abstraction layer and header-file-only class taking care about:
  • available interfaces and their management from configure method (e.g. this part of JTC)
  • reading from hardware (see read_state_from_hardware(JointTrajectoryPoint & state) method)
  • writing commands to hardware by abstracting logic about existing interfaces from here

Open questions about realization:

• naming: IMO, the therm "Kinematics" describes these interfaces. Any other ideas?
• Communication with controllers using JointTrajectoryPoint message?
• Should this class be part of the joint_trajectory_controller package?
  PROS: part of this repository; almost everyone is having JTC installed
  CONS: other controllers will depend on joint_trajectory_controller package
• Should this class be placed into controller_interface/helpers.hpp file?
  PROS: the package is already dependency of all controllers
  CONS: controllers logic is placed into ros2_control repository
• Adding a new package with abstractions/helper functionalities here?
  PROS: clean dependency tree and all logic inside the same repository.
  CONS: additional package

Start with the referenced joint_trajectory_controller.cpp sections for interface configuration, hardware state reads, and command writes, then compare controller_interface/include/controller_interface/helpers.hpp. Resolve the naming, message, package placement, and dependency questions before implementing; done requires an agreed abstraction and its adoption by the relevant controllers.