Added a vwarp implementation of the bottom-up kernel for the BFS stepwise algorithm
Optimized CPU and GPU gather functions of the hybrid BFS stepwise algorithm.
Updated the way the frontier list is allocated such that it is limited to a size that is a ratio of the GPU device memory
Refactored the top-down kernel of the stepwise BFS algorithm to avoid blocking
Merge pull request #4 from ahgharaibeh/hybrid-bfs-stepwise

Hybrid bfs stepwise