[Pid] Save i_term instead of error integral (backport #294)

[mergify]

The internal state should be the i_term to avoid jumps in the output while tuning the integral gain. Reverts #33

With constant gains, this PR does not change the current behavior of output calculation. Only the field in the published message and the result of get_current_pid_errors will be changed.

I got confused of the message definition, because it has some duplicates:
ros-controls/control_msgs#173
If yes, let's fix that in a new PR.

---

This is an automatic backport of pull request #294 done by Mergify.

[mergify]

Cherry-pick of 37a12e8 has failed:

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Changes to be committed:
	modified:   control_toolbox/include/control_toolbox/pid.hpp
	modified:   control_toolbox/include/control_toolbox/pid_ros.hpp
	modified:   control_toolbox/src/pid_ros.cpp
	modified:   control_toolbox/test/pid_tests.cpp

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   control_toolbox/src/pid.cpp

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

[christophfroehlich]

When we already break ABI with #386, then let's backport this also

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 88.88889% with 1 line in your changes missing coverage. Please review.

Project coverage is 74.98%. Comparing base (ee359df) to head (c953258).

Files with missing lines	Patch %	Lines
control_toolbox/src/pid_ros.cpp	66.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##            jazzy     #389      +/-   ##
==========================================
- Coverage   74.98%   74.98%   -0.01%     
==========================================
  Files          29       29              
  Lines        1391     1387       -4     
  Branches       86       85       -1     
==========================================
- Hits         1043     1040       -3     
  Misses        298      298              
+ Partials       50       49       -1     

Flag	Coverage Δ
unittests	74.98% <88.88%> (-0.01%)	⬇️

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

Files with missing lines	Coverage Δ
control_toolbox/include/control_toolbox/pid.hpp	77.77% <ø> (ø)
...ontrol_toolbox/include/control_toolbox/pid_ros.hpp	100.00% <ø> (ø)
control_toolbox/src/pid.cpp	81.00% <100.00%> (+0.23%)	⬆️
control_toolbox/test/pid_tests.cpp	100.00% <ø> (ø)
control_toolbox/src/pid_ros.cpp	62.84% <66.66%> (ø)

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