KD-tree classes and adaptors

// structs

template <class MatrixType, int DIM = -1, class Distance = nanoflann::metric_L2>
struct nanoflann::KDTreeEigenMatrixAdaptor;

// classes

template <
    typename Distance,
    class DatasetAdaptor,
    int DIM = -1,
    typename IndexType = size_t
    >
class nanoflann::KDTreeSingleIndexAdaptor;