Standardize spaningTree functions output

[cvvergara]

Currently pgr_drivingDistance has outputs:

(seq, depth, start_vid, pred, node, edge, cost, agg_cost)

For Kruskal's algorithm

• pgr_kruskalDD
• pgr_kruskalDFS
• pgr_kruskalBFS
  and for Prim's algorithm
• pgr_primDD
• pgr_primDFS
• pgr_primBFS

Output needs to be standardized.

Change:

(seq, depth, start_vid, node, edge, cost, agg_cost)

to (by adding pred)

(seq, depth, start_vid, pred, node, edge, cost, agg_cost)