Problems with tracers and MPI

[geodynamics]

Problems with tracers and MPI

[gassmoeller]

I have talked to Louise about this, and she will forward the problem to Eric, who has the most experience on the tracer code, so maybe there will be some improvement in the near future.

[ian-r-rose]

At the very least we may want to remove the assertion that the particle count never changes. I'm not sure it is realistic to expect that we never lose a tracer, especially where velocity gradients are sharp.

[gassmoeller]

This should finally be closed with #411. @maxrudolph do you want to check? I never encountered these problems during my recent tests on more than a thousand cores.
In the new version particles are allowed to get lost, although it almost never happens. The number of particles is tracked in the statistics file.

[maxrudolph]

@gassmoeller Using the development versions of aspect and deal.ii, it appears that this problem is solved! Thanks for contributing your vastly improved tracer code!