Points to volume should include density

As the title suggests, it's frustrating that the points to volume creates "flat" volumes. The volume density is clamped so that having more than one particle in proximity to others doesn't add cumulatively. Intuitively, it should be cumulative. This would allow us to build grids or volumes with properly varying density.

Note: the Alpha of 5.0 still doesn't have this facility (!?).

Work was done years ago for collecting neighbour groups, so building density fields can surely be based on that ? (ref: projects.blender.org/blender/blender/pulls/110675 )