Use OpenMP enabled FLANN to build codebook (vocab)

[insikk]

Many people use FLANN to do k-means clustering

When I tried python binding for the library, it uses only single core.

So I concluded it is slow somehow in python.

However, I cannot conclude FLANN is slow.

Things to do:

[ ] Measure time of large scale k-means clustering with FLANN both python binding version and pure c++ version

• refer pure c++ implementation here