Fixes to the gravity code

Merged Matthieu Schaller requested to merge gravity_fixes into master

This fixes #363 (closed). The long-range gravity task has to consider all the cells that were assigned to it at the rebuild time.

Additional improvements:

  • Drift the multipoles while activating the tasks,
  • Avoid FPEs in the FFT task,
  • Adjust the number of tasks per top-level required to run.

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