[feat] Add voxel grid filter

Add a voxel grid filter using the custom voxel grid based on an unordered_map to filter points. Added sampling options (first point found, last point found, max intensity, center point and centroid). The leaves of the voxel grid are uniform and described by a LeafSize. The bounding box of the voxel grid must be given as a parameter.

vtkVoxelGridProcessor: Add half leaf size offset to the bounds of the voxel grid to avoid on bounds data issues. Add voxel position vality test.