Given that I have a set of edges, e.g:

```
using Graphs
g = Graphs.grid([5, 5])
es = edges(g)
```

How do I create a new Graph, using those edges, but wit ha specific number of vertices? In this case I can do:

```
SimpleDiGraphFromIterator(es)
```

However, this does not permit to create graphs with vertices that are unconnected to an edge, e.g.

```
g = SimpleGraph(10)
es = edges(g)
SimpleDiGraphFromIterator(es)
```

creates an empty graph. Is there some way to specify that the graph should have a certain number of vertices? I can always create a new graph and add edges manually, but wanted to check whenever there was something better.