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 :warning:

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Additional details and impacted files

```diff @@ 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 ```

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