Remove the locking of cells when trying to activate the gpart drift. Moved the checks that the task actually exist into a debugging checks section.

Better version of the choice made to prevent extreme values of the distance crietrion in make_self_gravity_tasks()

This reverts commit 004fd34d.

Use the top-level cell and not the current cell in the long-range force task to decide what cell is in range.

Add some implicit tasks to walk the tree up and down after the multipole initialisation and before the downpass.