Create new conda/pip package for PropertyGraph

[rlratzel]

Who: end users, cugraph developers
What: Create a new package (cugraph and pip) for users with minimal dependencies needed only for core PG functionality
Why: The new PG implementation will likely have new dependencies not essential to other cugraph use cases, so a separate package benefits users by keeping dependencies for non-PG use cases to a minimum.

Questions:

• PG won't (likely) depend on cugraph, but should cugraph depend on PG, perhaps if we want cugraph.Graph to have support for properties like NetworkX Graph?
• If PG has a method to extract a cugraph.Graph subgraph, should it have a hard dependency on cugraph? If so, does that remove a major advantage of having PG and cugraph in separate packages (ie. not requiring PG to have all cugraph deps)?

Tasks

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1. Pick a name (probably one that matches dir name in source tree)
2. Update CI scripts to build new PG package and run newly-located tests
3. Add conda recipe, setup.py, etc. for new PG packages
4. Add/update docs for new PG package
5. Add/update notebook(s) for new PG package