Include parallel sort tasks in task dump and add support for MPI dumps

Dumps multiple task/thread/rank files for multiple time steps and provides scripting support to process these into graphics for each rank and timestep.

Matthieu, reasonably uncontroversial as only small changes to main.c to check. If you want to test use the new -y 1 flag to dump thread info files and use process_plot_tasks_MPI to create the graphics and a web page to view them.