new: invoke adbnx_adapter from nxadb.Graph constructor

[aMahanna]

Introduces the ability to rely on incoming_graph_data in an nxadb.Graph to insert into ArangoDB.

Useful for when we want to load NetworkX data into ArangoDB without having to deal with the NetworkX Adapter.

For now, we're just using the NetworkX Adapter under the hood. Long term, we could perhaps rely on arangoimport as an alternative...

Example:

import networkx as nx
import nx_arangodb as nxadb

G_nx = nx.karate_club_graph()
G_adb = nxadb.Graph(graph_name="karate", incoming_graph_data=G_nx, default_node_type="person")

# Graph "karate" is now loaded in ArangoDB