Add depth outputs from SPC raytracing, slight API refactor, fix raytrace AABB behaviour

[tovacinni]

Summary of changes:

• mark_first_hits is now mark_pack_boundary which a more general packed operation.
• SPC raytrace can now return depth. This deprecates the need for the old ray_aabb function.
• Some edge cases in raytracing is handled differently now. Before, rays could miss if they are exactly between 2 voxels, but now it will hit both.
• raytrace will now return 2 tensors, the ray index and the point hierarchy index. Previously these 2 tensors were stored as a single tensor, which was confusing.
• Other misc enhancements and fixes.

This also addresses #463 .

[Caenorst]

LGTM! thanks!