We now implement the `nodesBetween` primitive with a double DFS visit.
Specifically, we have:
1) A forward DFS that starts from the `Source`, and stops at `Target`.
2) A backward DFS that starts from the `Target`, and stops at `Source`.
The final result is then the intersection of the nodes found by the two
above DFS visits.
A new `findReachableNodes` primitive is also added to perform a DFS from
a `Source` node, which stops at the `Stop` parameter node, if present.
Added unit tests to improve the coverage.
Andrea Gussoni
e2090b8746