Make num_threads a paramter of the voxel hash map

For backwards compatibility, use a default value of "automatic"

cpp/kiss_icp/core/VoxelHashMap.cpp

@@ -44,8 +44,6 @@ struct ResultTuple {
     std::vector<Eigen::Vector3d> target;
 };
 
-constexpr int NUM_THREADS_ = 16;
-
 }  // namespace
 
 namespace kiss_icp {
@@ -97,7 +95,7 @@ VoxelHashMap::Vector3dVectorTuple VoxelHashMap::GetCorrespondences(
 
     using points_iterator = std::vector<Eigen::Vector3d>::const_iterator;
     ResultTuple correspondences(points.size());
-    tbb::task_arena limited_arena(NUM_THREADS_);
+    tbb::task_arena limited_arena(max_threads_);
     limited_arena.execute([&]() -> void {
         correspondences = tbb::parallel_reduce(
             // Range

cpp/kiss_icp/core/VoxelHashMap.hpp

@@ -26,6 +26,7 @@
 // should at least acknoowledge the work from CT-ICP by giving a star on GitHub
 #pragma once
 
+#include <oneapi/tbb/task_arena.h>
 #include <tsl/robin_map.h>
 
 #include <Eigen/Core>
@@ -52,10 +53,14 @@ struct VoxelHashMap {
         }
     };
 
-    explicit VoxelHashMap(double voxel_size, double max_distance, int max_points_per_voxel)
+    explicit VoxelHashMap(double voxel_size,
+                          double max_distance,
+                          int max_points_per_voxel,
+                          int max_threads = tbb::task_arena::automatic)
         : voxel_size_(voxel_size),
           max_distance_(max_distance),
-          max_points_per_voxel_(max_points_per_voxel) {}
+          max_points_per_voxel_(max_points_per_voxel),
+          max_threads_(max_threads) {}
 
     Vector3dVectorTuple GetCorrespondences(const Vector3dVector &points,
                                            double max_correspondance_distance) const;
@@ -70,6 +75,7 @@ struct VoxelHashMap {
     double voxel_size_;
     double max_distance_;
     int max_points_per_voxel_;
+    int max_threads_;
     tsl::robin_map<Voxel, VoxelBlock, VoxelHash> map_;
 };
 }  // namespace kiss_icp