vertex_path and edge_path should use a_star

[mtfishman]

vertex_path and edge_path should use a_star, which is now implemented for AbstractNamedGraph.

[mtfishman]

Also, we could consider more explicit names like shortest_path_vertices and shortest_path_edges, possibly with extensions like shortest_path_tree that outputs a directed graph representing the path.

[mtfishman]

@leburgel on a related topic, note that in #20 I added a wrapper for Graphs.steiner_tree, which finds the minimal spanning tree connecting multiple vertices (so in the case of two vertices, it should reduce to the shortest path between the two vertices). I was wondering if this is relevant for orthogonalizing a tree tensor network where the orthogonality center is a group of vertices towards a new center outside of that group, where I think a Steiner tree could help determine which vertices to orthogonalize. Maybe this is overkill for trees and you can just get the shortest path to one vertex in the orthogonality center region, but it seemed like steiner_tree must be useful somewhere...