ifPresent and ifAbsent methods #26
Description
The concept here is to add methods to Vertex similar to connectTo and connectFrom` but with a check.
connectToIfPresent(ID to) creates an edge from this vertex to to if to exists.
connectFromIfPresent(ID from) creates an edge to this vertex from from if from exists.
connectToIfAbsent(ID to) creates an edge from this vertex to to if to does not exist.
connectFromIfAbsent(ID from) creates an edge to this vertex from from if from does not exist.