Don't lock the tree for the grav_init task. Don't call the unsorted pairs at all. Skip the entire zeroing loop when no xparts in runner_do_sort()

Gravity multi dt

Some improvements to the gravity code:

 - Corrected typo in 4th order vector powers.
 - Use a user-defined opening angle for the distance checks.
 - Increase accuracy of the exact gravity calculation.
 - Only do exact gravity calculation if a matching result file is not present in the same directory.
 - Reinstate the dumping of individual task timers.
 - Added script to plot the accuracy of the calculation.
 - Crash if the M2L kernel is called with a length smaller than the softening length.

See merge request !331

Use short int consistently for unlocks. Check that the waits and unlocks don't overflow.

Two small changes:
 - `task->nr_unlock_tasks` is a `short int` so we should be consistent with the types in `scheduler_set_unlocks()`.
 - When debugging, check that we don't overflow the number of `wait` or `unlocks`.

As I am about to introduce some gravity tasks that will have a lot of unlocks I would like to be safe first.

See merge request !330

Improved the way of computing the exact accelerations by removing the need of recalculating exact forces that already exist

Use the multipole acceptance criterion to build the long-range interactions and not just the geometry of the mesh.