These are some improvements to the space structure to speed-up MPI calculations and the planetary things:
- We make a list of non-empty top-level cells,
- We make a list of local non-empty top-level cells,
- engine_drift_all() only loops over the local cells with tasks (instead of all the grid),
- space_split() only loops over the local non-empty cells,
- space_regrid() only loops over the local non-empty cells to figure out h_max.
- The top-level gravity task only loops over the non-empty cells.