Initial PropertyGraph implementation and tests

[rlratzel]

Initial implementation of the PropertyGraph graph class and corresponding unit tests.

pG = cugraph.experimental.PropertyGraph()

# Add properties
pG.add_vertex_data(...)
pG.add_edge_data(...)

# Select specific vertices and edges based on conditions
selection = pG.select_vertices("(user_location==47906) | (user_location==78750)")
selection += pG.select_edges("_TYPE_=='referrals'")

# Extract a subgraph using selection
G = pG.extract_subgraph(selection=selection,
                        create_using=DiGraph_inst,
                        edge_weight_property="review_rating")

# run some algo on G, get algo_results

# annotate results with properties
algo_results_with_properties = pG.annotate_dataframe(algo_results, G, edge_vertex_id_columns=("from", "to"))

# Add algo results back to the PropertyGraph for further analysis
pG.add_edge_data(algo_results, type_name="referral_data", vertex_id_columns=("from", "to"))

closes #1876

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[rlratzel]

rerun tests

[acostadon]

Added a simple test for a completely empty graph but not sure it is worth adding during burn down. Can commit and push if wanted.

[BradReesWork]

@gpucibot merge