refactor(kalman_filter): make getter functions explicitly return values

[IshitaTakeshi]

Description

Getter functions should explicitly return values.
Doing this allows the return values to be constants, and the readability improves.

Related links

Tests performed

No tests have been performed yet. The time-delay kalman filter has a large state so how to validate the internal state will be an open question.

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[codecov]

Codecov Report

Base: 10.27% // Head: 10.17% // Decreases project coverage by -0.10% ⚠️

Coverage data is based on head (69751b9) compared to base (87f8142).
Patch coverage: 0.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1895      +/-   ##
==========================================
- Coverage   10.27%   10.17%   -0.11%     
==========================================
  Files        1158     1255      +97     
  Lines       82151    90590    +8439     
  Branches    19120    20658    +1538     
==========================================
+ Hits         8441     9214     +773     
- Misses      64568    71958    +7390     
- Partials     9142     9418     +276     

Flag	Coverage Δ		*Carryforward flag
differential	5.15% <0.00%> (?)
total	10.16% <0.00%> (-0.10%)	⬇️	Carriedforward from 4485736

*This pull request uses carry forward flags. Click here to find out more.

Impacted Files	Coverage Δ
...mon/kalman_filter/src/time_delay_kalman_filter.cpp	0.00% <0.00%> (ø)
localization/ekf_localizer/src/ekf_localizer.cpp	0.00% <0.00%> (ø)
system/default_ad_api/src/routing.cpp	33.33% <0.00%> (-4.17%)	⬇️
...tion_utils/test/src/trajectory/test_trajectory.cpp	26.16% <0.00%> (-0.03%)	⬇️
perception/tensorrt_yolo/src/nodelet.cpp	0.00% <0.00%> (ø)
perception/lidar_centerpoint/src/node.cpp	0.00% <0.00%> (ø)
planning/obstacle_stop_planner/src/node.cpp	0.00% <0.00%> (ø)
common/osqp_interface/src/osqp_interface.cpp	34.35% <0.00%> (ø)
planning/route_handler/src/route_handler.cpp	0.00% <0.00%> (ø)
planning/rtc_interface/src/rtc_interface.cpp	0.00% <0.00%> (ø)
... and 299 more

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[kminoda]

LGTM

[kminoda]

@IshitaTakeshi Just a reminder, but I think you need to get a review from the code owners of kalman_filter (currently only @.yukkysaito, but maybe we should consider adding other people as a new maintainer, too)

[IshitaTakeshi]

@YamatoAndo is also added as a code owner. How about?

[kminoda]

@IshitaTakeshi I don't think so (at least from here)

  <description>The kalman filter package</description>
  <maintainer email="yukihiro.saito@tier4.jp">Yukihiro Saito</maintainer>
  <license>Apache License 2.0</license>

[IshitaTakeshi]

@kminoda I also could see the message "Waiting on code owner review from yukkysaito." on GitHub

[IshitaTakeshi]

@yukkysaito Could you review this?

[kminoda]

It still requires a review from @.yukkysaito even after merging 87f8142 ... 🤔

[kenji-miyake]

It still requires a review from @.yukkysaito even after merging 87f8142 ... thinking

@kminoda Please merge #1983 to reflect maintainers to CODEOWNERS.