Replace nanoflann

[koide3]

Replace nanoflann with new KdTree implementation for the following reasons:

• To resolve the conflict of nanoflann namespace with ones in other packages.
• To simplify the KdTree usage. OMP and TBB-based tree construction routines now return the same KdTree class.
• To improve the KdTree query speed. Note that it's not a major improvement. Only a few percent speed up comes at the cost of flexibility limitation (The new KdTree can only process 3D points).

EDIT: Task-based OpenMP parallelism causes memory issues with Eigen matrices on MSVC. To avoid it, disable parallel kdtree construction on MSVC.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 97.38562% with 4 lines in your changes are missing coverage. Please review.

Project coverage is 88.50%. Comparing base (86c2e40) to head (fd418bb).
Report is 2 commits behind head on master.

Files	Patch %	Lines
include/small_gicp/ann/knn_result.hpp	83.33%	4 Missing ⚠️

❗ Your organization needs to install the Codecov GitHub app to enable full functionality.

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #42      +/-   ##
==========================================
- Coverage   91.77%   88.50%   -3.28%     
==========================================
  Files          35       36       +1     
  Lines        1179      774     -405     
  Branches      182       70     -112     
==========================================
- Hits         1082      685     -397     
+ Misses         97       89       -8     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.