Add a section that introduces `mergeV` and `mergeE`

[krlawrence]

Significant improvements to the "create if not exist" pattern were introduced in TinkerPop 3.6.0, with refinements in subsequent releases, via the new mergeV and mergeE steps. Quite a bit of work will be needed to weave these into the manuscript. Work should include:

1. Adding a section that describes the new steps. Should be part of Chapter 4.
2. Updating the existing fold..coalesce examples, where appropriate, to mention the new merge steps.
3. Creating appropriate sample data for the examples in the book, and sample code.
4. Adding new sample code as appropriate (there is an MRG example that can help with this)