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 ⚠️

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Additional details and impacted files

@@            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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