GraphUri examples:
"tinker:": in-memory TinkerGraph
"tinker:/tmp/testGraphs/tGraph": on-disk TinkerGraph in absolute directory
"tinker:./testGraphs/tGraph": on-disk TinkerGraph in relative directory
"tinker:./testGraphs/tGraphML?fileType=graphml": on-disk TinkerGraph in GraphML format
See associated unit tests.