Metric (distance) classes

// structs

template <class T, class DataSource, typename _DistanceType = T>
struct nanoflann::L1_Adaptor;

template <class T, class DataSource, typename _DistanceType = T>
struct nanoflann::L2_Adaptor;

template <class T, class DataSource, typename _DistanceType = T>
struct nanoflann::L2_Simple_Adaptor;

struct nanoflann::metric_L1;
struct nanoflann::metric_L2;
struct nanoflann::metric_L2_Simple;