refactor!: Consistent naming of GSF mixture reduction and component merging

[benjaminhuth]

This PR applies consistent naming for two similar operations on Gaussian mixtures:

• component merging is the procedure of merge Gaussian components to a single set of parameters+covariance
• mixture reduction is reducing the number of components in a mixture

[codecov]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (cc29c08) 49.71% compared to head (f3d3855) 49.71%.

Files	Patch %	Lines
...re/include/Acts/TrackFitting/GaussianSumFitter.hpp	50.00%	0 Missing and 1 partial ⚠️
Core/include/Acts/TrackFitting/detail/GsfActor.hpp	87.50%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2658   +/-   ##
=======================================
  Coverage   49.71%   49.71%           
=======================================
  Files         474      474           
  Lines       26859    26859           
  Branches    12362    12362           
=======================================
  Hits        13352    13352           
  Misses       4706     4706           
  Partials     8801     8801           

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[github-actions]

📊: Physics performance monitoring for f3d3855

Full contents

physmon summary

• ✅ CKF truth_smeared
• ✅ IVF truth_smeared
• ✅ AMVF truth_smeared
• ✅ Track Summary CKF truth_smeared
• ✅ Seeding truth_estimated
• ✅ CKF truth_estimated
• ✅ IVF truth_estimated
• ✅ AMVF truth_estimated
• ✅ Track Summary CKF truth_estimated
• ✅ Seeding seeded
• ✅ CKF seeded
• ✅ IVF seeded
• ✅ AMVF seeded
• ✅ AMVF (+grid seeder) seeded
• ✅ Track Summary CKF seeded
• ✅ Seeding orthogonal
• ✅ CKF orthogonal
• ✅ IVF orthogonal
• ✅ AMVF orthogonal
• ✅ Track Summary CKF orthogonal
• ✅ Ambisolver seeded
• ✅ Ambisolver orthogonal
• ✅ Seeding ttbar
• ✅ CKF ttbar
• ✅ Ambisolver
• ✅ Track Summary CKF ttbar
• ✅ AMVF ttbar
• ✅ AMVF (+grid seeder) ttbar
• ✅ Truth tracking (GSF)
• ✅ Truth tracking
• ✅ Particles inital fatras
• ✅ Particles final fatras
• ✅ Particles inital geant4
• ✅ Particles final geant4

[acts-project-service]

🔴 Athena integration test results [6e8cd68]

🔴 Some tests have failed!

Please investigate the pipeline!

status	job
🟢	run_unit_tests
🟢	run_ci_tests: ../athena/AtlasTest/CITest/test/ActsBenchmarkWithSpot.sh 8 100
🟢	run_ci_tests: ../athena/AtlasTest/CITest/test/ActsWorkflow.sh
🔴	run_ci_tests: ../athena/AtlasTest/CITest/test/ActsValidateAmbiguityResolution.sh
🟢	run_ci_tests: ../athena/AtlasTest/CITest/test/ActsValidateTracks.sh
🟢	run_ci_tests: ../athena/AtlasTest/CITest/test/ActsValidateActsCoreSpacePoints.sh
🟢	run_ci_tests: ../athena/AtlasTest/CITest/test/ActsValidateActsSpacePoints.sh
🟢	run_ci_tests: ../athena/AtlasTest/CITest/test/ActsValidateSeeds.sh
🟢	run_ci_tests: ../athena/AtlasTest/CITest/test/ActsValidateOrthogonalSeeds.sh
🟢	run_ci_tests: ../athena/AtlasTest/CITest/test/ActsValidateClusters.sh
🟢	run_ci_tests: ../athena/AtlasTest/CITest/test/ActsPersistifyEDM.sh
🟢	run_ci_tests: ../athena/AtlasTest/CITest/test/ActsKfRefitting.sh
🟢	run_ci_tests: python3 ../athena/Tracking/Acts/ActsGeometry/test/ActsExtrapolationAlgTest.py
🟢	run_ci_tests: python3 ../athena/Tracking/Acts/ActsGeometry/test/ActsITkTest.py
🔴	run_workflow_tests_run4_mc
🟢	run_workflow_tests_run2_data
🟢	run_workflow_tests_run3_mc
🟢	run_workflow_tests_run3_data