Calculate frame difference

[francesco-donofrio]

This PR introduces a method to compute frame differences, along with a corresponding KDL implementation.

[aprotyas]

Why not imbibe the size constraints?

Suggested change

	std::vector<double> & x_a_vec, std::vector<double> & x_b_vec, double dt,
	std::vector<double> & delta_x_vec);
	std::array<double, 7> & x_a_vec, std::array<double, 7> & x_b_vec, double dt,
	std::array<double, 6> & delta_x_vec);

[francesco-donofrio]

I used std::vector<double> to maintain consistency with the convert_joint_deltas_to_cartesian_deltas method, where std::vector<double> is used for the delta_x parameter.

[codecov]

Codecov Report

Attention: Patch coverage is 80.00000% with 10 lines in your changes missing coverage. Please review.

Project coverage is 80.00%. Comparing base (f9c7c3a) to head (ac8a8c3).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
kinematics_interface/src/kinematics_interface.cpp	47.05%	6 Missing and 3 partials ⚠️
...ics_interface_kdl/src/kinematics_interface_kdl.cpp	94.11%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #93      +/-   ##
==========================================
- Coverage   80.20%   80.00%   -0.21%     
==========================================
  Files           4        4              
  Lines         197      245      +48     
  Branches       35       52      +17     
==========================================
+ Hits          158      196      +38     
- Misses         28       34       +6     
- Partials       11       15       +4     

Flag	Coverage Δ
unittests	80.00% <80.00%> (-0.21%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
...lude/kinematics_interface/kinematics_interface.hpp	100.00% <ø> (ø)
...terface_kdl/test/test_kinematics_interface_kdl.cpp	100.00% <100.00%> (ø)
...ics_interface_kdl/src/kinematics_interface_kdl.cpp	70.43% <94.11%> (+3.43%)	⬆️
kinematics_interface/src/kinematics_interface.cpp	65.90% <47.05%> (-11.87%)	⬇️

[christophfroehlich]

in principle LGTM.
But afaik this is really breaking ABI in the controllers due to the new virtual method.
Do you need this in Jazzy? We can break Rolling, but want to avoid that on Jazzy.

We can either add version macros (with rclcpp/version.h for example) to add this only on rolling or create a new branch for jazzy.

[francesco-donofrio]

@christophfroehlich I'm currently using such feature in Humble, into a custom controller.

What do you mean with breaking ABI in the controllers?

[christophfroehlich]

Ok we can't backport this to humble. But you always can compile the rolling development version of the ros2_control stack from source on humble distro.

Disclaimer: I'm not an expert here but from what I learned:
Changing/Adding virual methods changes the vtable layout. Programs statically linking against this library break if not both are updated accordingly. This happens often if users only update the ros2_controllers but don't upgrade the dependencies: Then the system may explode unexpectedly.