Replace colmap flann with nanoflann for knnSearch

[yuanxion]

No description provided.

[yuanxion]

With commit 35200c8:
nanoflann (search 1 single tree, ~ 0.030s/query) and flann (search 4 trees, ~0.287s/query).

As seen in above picture, the query (q, only list first 48 dim) is the same in flann and nanoflann, but the found 2 nearest neighbors (d, only list first 48 dim) are not the same (the indices and the distances are listed at the end of the lines). 

The running time of nanoflann is fast, but the results are different with flann.

This causes the later process failed (WARNING: No images with matches found in the database).