Dev/gremlin valuemap - #250

g2 = (g
  .neptune(**creds)
  .gremlin('g.E().limit(10)') # fetches nodes, edges
  .fetch_nodes()  # fetch node props
  .fetch_edges()) # fetch edge props

g2.plot()

Add:

• fetch_edges: gremlin query for properties of current edges via g.neptune(...).gremlin(...).fetch_edges()

Fix:

• fetch_nodes: do not wipe out edges if not None, e.g., fetch_nodes().fetch_edges() == fetch_edges().fetch_nodes()
• Switch to g.V().elementMap() / g.E().elementMap() for fetch
• Work around cosmos <> gremlin graphjson mismatches on neptune side. Must still revisit for enriching for cosmos gremlin dialect