Consider MST with backtracking to check budget criteria

[chrisnatali]

Rationale: Since an MST is guaranteed the minimum network required to connect all nodes, why not start with that and prune edges/paths if the two-way budget criteria is not met by any 2 end-nodes of a path including that edge? This way, we work back from what's known to be optimal. It would resolve the following case more elegantly:

modKruskal/modBoruvka:

new backtrack approach:

It may also be faster and simpler to rationalize.