HDX/AV: edge extremes search based on adjacency lists

[jteresco]

Idea: have variants on the edge search that uses the edge list (current) or by traversing adjacency lists (which also exist).

[jteresco]

This would be really nice for http://courses.teresco.org/cs385_s19/labs/metal-intro/, aiming to have it in place by next offering of Algorithms.

[jteresco]

This could be framed as a new option on the edge search AV, or on the traversals/spanning trees AV (which already has the adjacency list-based traversal capabilities).

[jteresco]

The new option on the edge search AV probably makes more sense here to show how an algorithm that needs to visit all edges would do so when the graph structure is an adjacency list.