Allow "IF NOT EXISTS" Option When Creating Index

[zach-blumenfeld]

Currently, create_vector_index will fail if the index already exists. I also don't have a method in this package, AFAIK, to check if an index already exists.

I think it's important for ingest programs to not terminate in these cases (i.e. running a recurring doc ingest into the same Neo4j DB). Can we have an argument or additional method to create an index "if not exists"