Separate map/flatmap coverage from coverage of lambda functions.

[krlawrence]

Currently the map and flatMap coverage is nested inside the coverage of Groovy lambdas. That made sense when just discussing TinkerGraph but now more and more people connect to a Gremlin Server it would make more sense to have map and flatMap moved up a level and just cover lambda specific examples in their current location.

Also add an example of checking an edge property inside a flatMap but only passing the incident vertex onto the next step.