Note the thread IDs seem offset to those in the plots, so that needs looking at sometime

Split the star formation task into two parts: The star formation bit and the resorting bit. Move the sf_count communication to just depend on the first one in order to speed things up.

Added communication functions to propagate the new number of star particles after a SF event has occured.

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.