* Add FlannIndex abstract interface and implement NanoFlannIndex subclass
* Refactored: made NanoFlann a new NN type instead of inheriting FlannIndex. Added tests. Vendoring nanoflann.h directly in the repo. RegistrationVis now use NANOFLANN_INDEX_KDTREE_SINGLE (instead of FLANN_INDEX_KDTREE_SINGLE) flann index for 2d points matching.
* cleanup comments, added FlannIndex doxygen
* Fixing windows tests
* updating flaky test
* Simplified interface, added flann kdtree single approach selectable by parameters.
* RegVis: symmetry of nanoflann for two branches of guess feature matching
* cv::BFMatcher baseline
* Small cmake optimization FLANN_KDTREE_MEM_OPT only defined for FlannIndex
* Refactored where FLANN_KDTREE_MEM_OPT is defined
* fixed file name already exist
* cleanup
* fixup build
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Co-authored-by: matlabbe <matlabbe@gmail.com>
