engine_step now writes to a file called timesteps.txt that contains timestep information along with a header.

the super cell is either a non-empty leaf or the first cell for which an interaction task is defined. need to add a condition for gravit tasks once these have been settled.

add a third communication step which exchanges the particles after the kick, just to make sure we have the correct ti_end_min values for each foreign cell.

link the send/recv tasks to all their cells and sub-cells. first step in linking send/recv tasks to density/force tasks.

Better output of wallclock time when writing a snapshot. Slight improvement to the task plotting scripts

in the cpuid map. These are now offered in numa node order rather
than core order. This means that hyperthreads should be picked last.

With this fix we look to work well with Intel MPI using multiple ranks
on the same workstation, but PLATFORM MPI remains uncooperative.
To help with that a new command-line option -a has been added to
disable processor affinity, even when the numa policy is set
and available.