Make Gremlin traversal source configurable

[michaelnchin]

Issue #, if available: #206

Description of changes:

• Add new section to %%graph_notebook_config input for Gremlin-specific options
• Support for specifying the traversal source to use for Gremlin queries, in case the name of the traversal source differs from the previously hardcoded default of g.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[krlawrence]

Something to consider is that not all Gremlin servers will be accessible by a user and therefore they will not have control as much over the naming of the "GraphTraversalSource" object (a.k.a. g). We should think through whether we want to allow all users to have the option to change this and have the backend database tell them via an error message that it is not allowed or explicitly prevent it as part of our parameter processing on the graph-notebook side.

[michaelnchin]

Thanks for the feedback @krlawrence. Hard checks have been implemented to block traversal_source from being changed when using a Neptune host, as is currently done for the SPARQL path config option. For all other hosts, we will catch the GremlinServerError returned after the fact, and output a more helpful error message.