new: fully support parameterized db object

[aMahanna]

There are limitations with using the NetworkX Config objects to store connection credentials, as that prevents the user from having multiple connections to different ArangoDB databases in one Python session.

This PR minimizes the use of the NetworkX Config object in favour of storing connection information on a per-Graph-object basis.

Related to #68

In other words:

from arango import ArangoClient
import nx_arangodb as nxadb

db_1 = ArangoClient("http://my_first_deployment:8529").db(...)
db_2 = ArangoClient("http://my_second_deployment:8529").db(...)

G1 = nxadb.Graph(name="MyGraph", db=db_1)

G2 = nxadb.Graph(name="MyGraph2", db=db_2)

Also introduces support for passing use_gpu to NetworkX Algorithms, e.g nx.pagerank(G1, use_gpu=False)

[aMahanna]

Setting environment variables (i.e DATABASE_HOST, DATABASE_USER, etc.) is still supported, but passing a custom db object will take priority over env vars