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

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

Re-introduce the 'end_force' function as a task in order for the cooling function to be called at the right place in the time-step.

Keep maintenance down.

The task plot analysis scripts now also displays the number of particle updates done in that time-step. It also shows the time distribution for pairs sorted by SID.