registration: parallelize SAC-IA error metric with OpenMP

[zeel2104]

Summary

This PR parallelizes the error accumulation loop in SampleConsensusInitialAlignment::computeErrorMetric() using OpenMP.

The change uses an OpenMP parallel for with a reduction(+ : error) and thread-local nearest-neighbor buffers via firstprivate(...), so the accumulated registration error can be computed in parallel while preserving the original behavior.

Changes

• parallelized the per-point loop in registration/include/pcl/registration/impl/ia_ransac.hpp
• added OpenMP reduction for the error accumulator
• kept nearest-neighbor index/distance buffers thread-local inside the parallel region

Validation

Built and ran the existing SAC-IA registration test locally on Windows/MSVC:

cmake --build D:\pcl\build --config Release --target test_sac_ia
ctest --test-dir D:\pcl\build\test -C Release -R sac_ia --output-on-failure

[mvieth]

Thank you for the pull request, I have two small comments.

[mvieth]

Suggested change

	const auto tree = tree_;
	const auto& tree = tree_;

I believe a reference would be a good choice here.

[mvieth]

Suggested change

	firstprivate(nn_index, nn_distance) reduction(+ : error)
	firstprivate(nn_index, nn_distance) reduction(+ : error) num_threads(threads_)

Please add the option to easily adapt the number of threads via setNumberOfThreads, see https://github.com/PointCloudLibrary/pcl/blob/master/features/include/pcl/features/impl/normal_3d_omp.hpp#L48 and https://github.com/PointCloudLibrary/pcl/blob/master/features/include/pcl/features/normal_3d_omp.h (but no need to also add chunk_size -- the default schedule is fine for now).

[zeel2104]

@mvieth
Addressed both comments:

• changed const auto tree = tree_; to const auto& tree = tree_;
• added setNumberOfThreads() / threads_ and wired num_threads(threads_) into the SAC-IA OpenMP loop

Validation:

• cmake --build build --config Release --target test_sac_ia
• ctest --test-dir build/test -C Release -R sac_ia --output-on-failure
• cmake --build build --config Release --target pcl_registration