Supporting 3D cell segmentation

[lambdamoses]

Definitely not in a rush since 3D cell segmentations are still uncommon. But the ones I have seen have 2D segmentations in a z-stack which are evenly spaced. There are actually Simple Feature geometries for 3D solid bodies, like polyhedron, I really don't want to deal with polyhedrons since it's really confusing to specify the coordinates. I think I can use MULTIPOLYGONs to represent the multiple z-planes. Then create a new S3 class inheriting from sf to have a field specifying the z spacing. This way geometric operations can still be performed, though only in x and y, but I think that should be good enough for quite a while since thick sections (not serial sections) are still uncommon.