Remove the check preventing max_dt to be larger than the total run time when running with cosmology.

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.