[APP] Parallel PR-Nibble

[bkj]

Implemented Parallel PR-Nibble.

This should be substantially simpler than the original gunrock version or @neoblizz's work in progress #69

Note: This matches the reference implementation from SDH, which is parameterized slightly different than the HIVE version. I need to dig up the HIVE reference code to make sure this is right, but it's definitely close. Maybe need to change the formulation slightly.

Observations:

• Uses the batch operator, but I think maybe we're doing lots of small kernel launches. The essentials version actually isn't any faster than a single threaded (!) CPU implementation on jhu.mtx, with the parameters that are hardcoded now. I think this is because the algorithm doesn't actually do a whole lot of work (in Pagerank terms, it doesn't spread mass to enough nodes). If you reduce alpha to 0.05 or 0.01 runtime goes up (less strongly local / more global diffusion) and the essentials version beats the CPU version.

Reference: Fig 5 + 6 in this paper -- https://arxiv.org/pdf/1604.07515.pdf

[neoblizz]

Gonna merge this as is and then look to see how we can improve the batch operator for the inner-loop.