Draft: Task modifications to account for "all the way down" void cell tree
This includes the modifications needed to make the tasks work with the void tree going the whole way down.
- Only bkg -> bkg self and pair tasks are ever made directly.
- Long range gravity only ever needs to care about background cells (and thus simplifies a lot).
- Some recursions need to exit “when not in void” not just “when in zoom”, the void timestep collect has been fixed to account for this here.