Adding more flags to control the new FLANN search engine

[smauq]

Added three new important flags to control the FLANN search engine for float descriptors:

• flann_num_checks: Controls the number of checks (time) spent on searching through the underlying KD-trees that are built. Especially for bigger maps larger values are necessary. This will slow down the search, but will simplify the RANSAC problem afterwards and the default parameters from BRISK will be relatively good.
• flann_eps: The OpenCV documentation is a bit lacking on this, but I assume the tolerance for when a descriptor is considered a nearest neighbor. Theoretically increasing this should speed up the search as it can exit earlier from a branch and not have to go all the way down.
• lc_flann_num_kdtrees: Number of parallel KD-trees to build and search. Around 4-6 seems to be the optimum.

[ethzasl-jenkins]

Test FAILed.

[ethzasl-jenkins]

Test PASSed.

[ethzasl-jenkins]

Test PASSed.

[smauq]

Made obsolete by https://github.com/ethz-asl/maplab/pull/384