Charm++ reductions

[jbakosi]

From http://charm.cs.illinois.edu/news.html#116:

Reductions over groups and chare arrays that apply commutative, associative operations (e.g. MIN, MAX, SUM, AND, OR, XOR) are now processed in a streaming fashion. This reduces the memory footprint of reductions. User-defined reductions can opt into this mode as well.

Explore how this can benefit us. Both walker and inciter could benefit. As this should work with user defined reductions, accumulation of PDFs in walker could potentially benefit. This is in Walker/Collector.C.