Shortest path for direct acyclic graphs (DAG)

Having the longest path algorithm for directed acyclic graphs means that we also have the shortest path algorithm for DAG.

```
function dag_shortest_path( g::::IsDirected, distmx::AbstractMatrix=weights(g); topological_order=topological_sort_by_dfs(g), ) = dag_longest_path( g, -distmx; topological_order )
```