Separating zoom and background `space_subdepth_diff_grav` treatments
This MR splits space_subdepth_diff_grav
such that its value can be tuned separately for zoom and background cells. This is done by introducing a new helper function in cell.h
which wraps the necessary logic for this decision and replaces anywhere the space_subdepth_diff_grav
logic appeared.
In addition, an extra printout has been introduced during task counts which will only be output during zoom simulations to display the counts of pair tasks between different types of cell.
Currently in draft because its untested while the machine is down but I thought I'd open it for comment.