Add concurrent kd-tree build perf test

[Mintpasokon]

Related Issue:

jlblancoc/nanoflann#165

What's new:

Added two performance test:

1. nanoflann build time vs. thread count
2. build speedup ratio vs. thread count

Files added:

1. test_n_thread_build.cpp to do the perf test.
2. analyze_thread_count.m to generate two plot.
3. stats_raw_results-27_jun_2023.tar with result txt generated by test_n_thread_build.cpp
4. perf5_build_time_VS_thread_count.pdf, based on my test on i7-12700KF, DDR5 4800.
5. perf6_speed_up_VS_thread_count.pdf, based on my test on i7-12700KF, DDR5 4800.

Files modified:

perf-tests/nanoflann/Makefile to compile test_n_thread_build.cpp.

I'm not very good with linux, so I didn't successfully install all the dependencies for this project, and I tested it by manually compiling test_n_thread_build.cpp using g++ ../perf-tests/nanoflann/test_n_thread_build.cpp -o test_n_thread. Although I extended the makefile, I did not compile with this makefile, so there may be some incompatibilities.

If you have any suggestions, please let me know. You could also re-run the tests and overwrite the pdf I submitted, if necessary.

[jlblancoc]

Thanks! 👍