Voxelize line segments instead of points

[Helveg]

Currently, when points are sparse in a morphology, voxelizing it, might leave in between voxels empty, depending on dynamically determined grid size: e.g. if we end up with voxels of 10um, and 2 points are 30um apart, 1 10um voxel in between them, will remain empty, while the segment that travels between the 2 points passes through it.

[Helveg]

• Use titular solution (check the line segment)
• Add an always-on interpolation step (costly overhead)
• Allow a flag to be set for interpolation (users might not know they have to, document it with a warning in future morphology voxelization docs)
• Do it automatically for morphologies used by VoxelIntersection (users might be surprised that a connection strat changes their morphologies)