[WIP] MNMG Cython / Python support and infrastructure.

[aschaffer]

This tracks work on items Pattern Accelerator (PA); BFS; SSSP Cython and Python infrastructure.

For more see from https://docs.google.com/document/d/1c6JjLgi0GfkZqRZoQOwLYu_5jq6IqEGrfk09qR3rUqk ("Andrei" suffix).

Dependencies: #1124

[GPUtester]

Please update the changelog in order to start CI tests.

View the gpuCI docs here.

[rlratzel]

I believe the work for this is now covered in #1163 and #1178 - should this PR be closed?
My mistake, I didn't realize from the title that this also includes changes for pagerank and others. However, PRs #1175 and #1177 are also open so we'll need to coordinate these.

[aschaffer]

Per latest discussions, the work in here has been incorporated in other PR(s), and is to be closed.