Generalize Sided combiners

[Shfty]

• Conditional expression should be customizable
  • i.e. Able to read from a third field to pick between Left / Right
  • Enables space partitioning
  • ex. If conditional field is a plane, will split space into Left / Right fields based on sidedness
  • Should be able to fold several partitioning SDFs, and apply ascending IDs to allow instancing-like behaviour