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