KD-Tree construction of visual

// structs

template <
    typename distance_t,
    class metric_t = nanoflann::L2_Simple_Adaptor<uint8_t ,     detail::TSIFTDesc2KDTree_Adaptor<distance_t>, distance_t>
    >
struct mrpt::vision::TSIFTDescriptorsKDTreeIndex;

template <
    typename distance_t,
    class metric_t = nanoflann::L2_Simple_Adaptor<float ,       detail::TSURFDesc2KDTree_Adaptor<distance_t>, distance_t>
    >
struct mrpt::vision::TSURFDescriptorsKDTreeIndex;