Speed-up for large top-level grids

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.