improve knn performance of voxelmaps

[koide3]

• Improve the performance of knn_search of IncrementalVoxelMap with better nearest neighbor result management.
• Support several voxel search patterns (DIRECT1/7/27)

[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 80.39216% with 10 lines in your changes missing coverage. Please review.

Project coverage is 93.34%. Comparing base (fccb619) to head (b2bed0c).

Files	Patch %	Lines
include/small_gicp/ann/incremental_voxelmap.hpp	73.68%	10 Missing :warning:

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #79 +/- ## ========================================== - Coverage 94.32% 93.34% -0.98% ========================================== Files 38 38 Lines 846 857 +11 Branches 79 77 -2 ========================================== + Hits 798 800 +2 - Misses 48 57 +9 ```

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