New functions to retrieve traversed graph elements

[lvca]

We'd need 3 new functions:

• traversedVertex, to return the traversed vertex
• traversedEdge, to return the traversed edge
• traversedElement, to return any of the traversed element

All functions should get 2 params:

• index, as starting index. negative numbers means from the end, positive from the beginning of the stack. Example 0 = absolute first traversed vertex, -1 = relative last one
• items, as optional, by default is 1, but specifying >1 collects the elements in a collection.

Example:

select $path, traversedVertex(-1) from ( traverse out() from #9:1 while $depth < 10 )