Implemented duplicated keypoint removal.
Implemented duplicated keypoint removal.

This is problem only with our hack-ish unoriented SIFT, because
we take the detected SIFT keypoints and strip their angle information,
which may lead to duplicated keypoints where there were previously
otherwise-identical keypoints with different angles.

rokm/alphagamma-descriptor,rokm/alphagamma-descriptor,rokm/alphagamma-descriptor