Modified write_to_neo (and generate_data) to allow one to specific an edge property to use as the relation name

[erickpeirson]

If edge_rel_key is not provided, then defaults to edge_rel_name. This involved making some existing parameters optional, in order to maintain the call signature and not impact compatibility.

For example:

from neonx import write_to_neo

import networkx as nx
G = nx.Graph()
G.add_nodes_from([1, 2, 3])
G.add_edge(1, 2, label='KNOWS')
G.add_edge(2, 3)

results = write_to_neo("http://localhost:7474/db/data/", G, 'LINKS_TO', 'Node', edge_rel_key='label')