Implement spatial partitioning machinery

[Shfty]

Need a generalized way to partition space, comparable to Repeat and its REPEATID* properties, but using arbitrary fields + combine to create a voronoi diagram of indexed cells that can be used by Select etc.

The index field is effectively a regular field, composed using the same union etc operators, but with an auto-incrementing index parameter that can be used in downstream conditionals.

The auto-incrementing will likely need to happen inside a combinator, and the index field will need to write into a context before its consumer.

This is likely best implemented as a pre modifier which composes a DynAsIR index field, and a Select statement comparable to the existing Repeat implementation.

[Shfty]

Implemented via voronoi field function pre-modifiers.