process: add voxel-grid downsampling with mode selection

[iosefa]

• Introduced voxelgrid_cell and voxelgrid_mode options for voxel-grid downsampling in tile processing.
• Supports "first" and "center" modes for representative point selection.
• Includes handling for empty tiles post-downsampling and graceful error handling per tile.
• Updated tests to cover voxel-grid thinning scenarios, including empty outputs.