Gravity splittask (non-MPI)

This implements task splitting for gravity. Changes are:

• Split large gravity tasks (self and pair)
• Replace gravity pair tasks that are far enough with a multipole-multipole task.
• Re-order operations in a time-step.

The last item has been a long-running accuracy issue as we were always running a time-step after detecting that a rebuild is needed.

The task-splitting is currently not enabled over MPI. I will make this work alongside the periodic gravity over MPI in the next merge request.

examples/EAGLE_12/eagle_12.yml

@@ -24,7 +24,6 @@ TimeIntegration:
   
 Scheduler:
   max_top_level_cells:    8
-  cell_split_size:        50
 
 # Parameters governing the snapshots
 Snapshots: