Conflict of nanoflann namespace issue

[LimHyungTae]

Hello Koide, thanks for sharing this nice stuff.

I tried to include this repository, and I found that once the repository already contains nanoflann.hpp, then two nanoflann.hpp brings namespace conflict error.

Of course, it's very easy to resolve by manually changing the workspace name in the own package, but maybe we have to change the namespace of nanoflann.hpp in this repository for better usability (e.g., namespace koide_nanoflann or something)

Thanks!

[koide3]

Thanks for reporting the issue. I recently re-implemented KdTree, and I'm considering to replace nanoflann with the new KdTree. It would resolve the conflict and bring (very slight) speed improvement.

[koide3]

I replaced nanoflann with a new KdTree implementation in this PR #42 that would resolve the conflict. I'll merge it once some errors on windows are fixed.