refactor(ekf_localizer): isolate mahalanobis functions and add tests

[IshitaTakeshi]

Description

• Isolate the Mahalanobis gate function
• Add a function to compute the squared Mahalanobis distance
• Add unit tests

Related links

Tests performed

• Validate the squared Mahalanobis distance
• Validate the gate condition around the boundary value

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[codecov]

Codecov Report

Base: 10.43% // Head: 10.41% // Decreases project coverage by -0.01% ⚠️

Coverage data is based on head (3934da4) compared to base (ce3ecfb).
Patch coverage: 14.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1877      +/-   ##
==========================================
- Coverage   10.43%   10.41%   -0.02%     
==========================================
  Files        1232     1235       +3     
  Lines       87941    88144     +203     
  Branches    20428    20592     +164     
==========================================
+ Hits         9174     9181       +7     
- Misses      69433    69600     +167     
- Partials     9334     9363      +29     

Flag	Coverage Δ		*Carryforward flag
differential	18.52% <17.50%> (?)
total	10.41% <0.00%> (ø)		Carriedforward from ce3ecfb

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

Impacted Files	Coverage Δ
..._localizer/include/ekf_localizer/ekf_localizer.hpp	0.00% <ø> (ø)
...on/ekf_localizer/include/ekf_localizer/warning.hpp	0.00% <0.00%> (ø)
localization/ekf_localizer/src/ekf_localizer.cpp	0.00% <0.00%> (ø)
...calization/ekf_localizer/test/test_mahalanobis.cpp	44.44% <44.44%> (ø)
localization/ekf_localizer/src/mahalanobis.cpp	60.00% <60.00%> (ø)

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[kminoda]

LGTM
(A DEBUG_INFO log defined in the mahalanobisGate function in original code is going to be deleted with this PR, but I think it won't be a big problem)