This example tests performance of InsertionSortCollider (with and without stride) and
SpatialQuickSortCollider and was basis for http://yade.wikia.com/wiki/Colliders_performace,
which features now-removed PersistentSAPCollider as well.

To run the test, say:

 yade-trunk-multi perf.table perf.py

It will take time to finish. To collet results and get graphs from the generated log files, say

 python mkGraph.py *.log

To see other results, log files are named like perf.64k.q.log (64k spheres with
SpatialQuickSortCollider), perf.32k.i.log (32k spheres, InsertionSortCollider),
perf.40k.is.log (InsertionSortCollider with stride) etc.

1. File with nSpheres spheres (loose packing) is generated first, it if doesn't exist yet.
   This can take a few minutes, but is done only the first time for the particular sphere
	number.

2. First iteration on the scene (TriaxialTest and the selected collider) with timings is
   done and timing.stats() printed (appears in the log file).

3. Another 100 iterations are measured with timing.stats(), after which the test exits.

(Note: InsertionSortCollider with stride is currently commented out in the table,
as striding is not effective until later during the simulation.)

