runner_do_sort now only looks at the do_sort and do_sub_sort flags set in the cell, not quite sure if we still need the cleanup flag.

re-factor the sorts activation into a single function to avoid repetition. turns out this is where the drift-before-sort bug was hiding, i.e. the missing drift activation in the for-loop.

Updated the way we converge towards the smoothing length. We now use a Newton-Raphson scheme. This applies to the Gadget-2 scheme.