Add FlannIndex abstract interface and implement NanoFlannIndex subclass (#1744)

* 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

---------

Co-authored-by: matlabbe <matlabbe@gmail.com>

corelib/src/CMakeLists.txt

@@ -131,7 +131,8 @@ SET(SRC_FILES
     rtflann/ext/lz4.c
     rtflann/ext/lz4hc.c
     FlannIndex.cpp
-    
+    nanoflann/NanoFlannIndex.cpp
+
     #clams stuff
     clams/discrete_depth_distortion_model_helpers.cpp
     clams/discrete_depth_distortion_model.cpp
@@ -872,6 +873,14 @@ add_definitions(${PCL_DEFINITIONS})
 
 # Add binary that is built from the source file "main.cpp".
 # The extension is automatically found.
+# rtflann's config.h is generated, as it is upstream, so that the
+# FLANN_KDTREE_MEM_OPT option travels in a header instead of a compile
+# definition: toggling it then recompiles the sources including rtflann rather
+# than every object file of the library (see the header for the details).
+configure_file(
+        ${CMAKE_CURRENT_SOURCE_DIR}/rtflann/config.h.in
+        ${CMAKE_CURRENT_BINARY_DIR}/rtflann/config.h)
+
 ADD_LIBRARY(rtabmap_core ${SRC_FILES} ${RESOURCES_HEADERS})
 ADD_LIBRARY(rtabmap::core ALIAS rtabmap_core)