change title, it's a bit of a mouthful but more descriptive of what we're actually going to talk about.

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.

See merge request !70

Overlapping tasks

This is a back-port of some changes I made to QuickSched: instead of taking the first lockable task with the largest weight, look for a task that maximally overlaps with the previously executed task.

This is done to maximize cache re-use, i.e. tasks with similar priorities operating on similar data will be scheduled closer to each other. I was already trying to do this by favouring tasks with the same super-cell as the previous task, but that was a bit of a mess. This should work much better.

Peter, can you check this both for correctness and if it doesn't cause a performance regression? I don't really expect a measurable performance gain directly, but this will have a strong effect on some caching that @alepper is currently working on.

Cheers!

See merge request !75

conflict in src/scheduler.c.

init tasks and tasks_ind to make sure they're not freed.

Fixes the bungled push to master I did just recently.

See merge request !74