[WIP] Speed-up cell_unskip_tasks

So this is a bit embarassing... Turns out that in cell_activate_subcell_tasks for pairs we were potentially recursing all the way down the tree before checking if anything is active at all.

Moved the check for activity to the top of the recursion, as it is done for the subcell-task interaction functions, this is already a bit faster on my laptop without breaking anything obvious. Should also improve parallel performance since this part of the code scales badly on the small timesteps.