[FEA][NV] Move nvgraph repo in cugraph repo

[afender]

This task will result in open sourcing nvGraph. NvGraph contains the core CUDA code for Jaccard, Louvain, BFS, SSSP and more.

The goal is to open source all at once to facilitate the integration and minimize the refactoring effort.

Before moving nvGraph repo here, the following tasks must be done in nvGraph:

• Use cmake infrastructure (see [FEA] Modify nvgraph to build with cmake #34)
• Review for IP
• Integrate recent changes from perforce (if any)

Then :

• Copy nvgraph code in cugraph repo: code should be in a nvgraph folder under cpp/nvgraph
• Add build option to produce nvgraph library
• rename nvgraph stand alone library nvgraph_st into nvgraph_rapids
• Update cugraph infrastructure to build the newly added nvpraph code

Important improvement

• Remove multiple CUB dependencies (3 versions of cub in cugraph after transferring nvgraph...)
• Reduce build time (nvgraph takes minutes to build, this will negatively impact cugraph development)

[afender]

Open sourcing now would reduce the deployment overhead and remove an unnecessary complexity layer in cugraph. Currently, nvgraph has to be updated on anaconda.com each time something change which is a source of bugs and slows down development.