Benchmarking of thread scalability with MPI

To be tested:

• swift_mpi on the EAGLE_25 box compiled with "none" as external potential and with metis.
• Run on 4 nodes with 1 to 16 threads per rank.
• Run with -g to get the overheads from gparts.

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• Total time broken down into:
  • engine_collect_timestep()
  • engine_launch()
  • engine_unskip()
  • engine_drift_all()
  • engine_rebuild()
  • engine_repartition()
  • engine_print_stats()
  • scheduler_reweight()

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• engine_repartition() broken down into:
  • partition_repartition()
  • engine_redistribute()
  • engine_makeproxies()

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• engine_rebuild() broken down into:
  • space_rebuild()
  • engine_maketasks()
  • engine_marktasks()

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• space_rebuild() broken down into:
  • space_regrid()
  • space_parts_get_cell_index()
  • space_gparts_get_cell_index()
  • engine_exchange_strays()
  • space_parts_sort() <-- Note this is also called in engine_redistribute(). Only want this call for now.
  • space_gparts_sort() <-- Note this is also called in engine_redistribute(). Only want this call for now.
  • part_relink_gparts_to_parts() <-- Note this is also called in space_split(). Only want this top-level call for now.
  • part_relink_parts_to_gparts() <-- Note this is also called in space_split(). Only want this top-level call for now.
  • space_split()