The 'createIfMissing()' methods should be removed from the Graph API

This issue stemmed from DNA-443. The existence of the "createIfMissing()" methods that exist only on the Graph interface (and not the Graph.Batch interface) mask the existence of newer, more powerful methods that can do the same thing on Graph and Graph.Batch:

graph.create(path).ifAbsent()...

or

batch.create(path).ifAbsent()...

In fact, this style makes available several other methods than "ifAbsent()", such as "orUpdate()", "byAppending()", and "orReplace()". These correspond to the different NodeConflictBehavior options.

Unfortunately, the 'create()' methods just return the Conjunction interface, while the 'createIfMissing()' methods return the "GetNodeConjunction" interface (which extends Conjunction and adds the "andReturn():Node" method). And there is at least one place where 'createIfMissing(...).andReturn()' is being called. Looking at this now, I'm not sure why the Graph.create(...) methods can't return the GetNodeConjunction interface. This would allow the existing uses of the 'createIfMissing(...).andReturn()' to replace them with 'create(...).ifAbsent().andReturn()' calls, and enable the removal of the 'createIfMissing(...)' methods.