Replace the absolute depth criterion for gravity tasks with a relative criterion based on the local depth of the tree.

Fixes and documentation of a few more places where the implementation will need to be updated in the future.

Also tag all top-level cells with foreign partners, do the multipole exchange after maketasks so that we have all the necessary tags available

Add some additional checks as well

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