Collect histogram of process loads (modeled and raw) in LB data reduction

[PhilMiller]

There's already an implementation of an approximate histogram for the diagnostics. Using that in the process stats reduction would allow quantile estimation, with potential uses in LB strategies.

[PhilMiller]

One possible use would be to iteratively focus refinement-based load balancing on the most-overloaded processes, as judged by their place in the ranking, rather than their absolute degree of overload which is the only thing they can compute with available stats right now.

[PhilMiller]

https://github.com/DARMA-tasking/vt/blob/develop/src/vt/utils/adt/histogram_approx.h

http://www.jmlr.org/papers/volume11/ben-haim10a/ben-haim10a.pdf

[lifflander]

@ppebay Are you still planning on doing this?