as.neuron should work for an igraph made from an ngraph

```r
ng=as.ngraph(Cell07PNs[[1]])
ng2=igraph::delete_edges(ng, 1:3)
n=as.neuron(ng2)
```
```
Error in as.igraph.vs(graph, v) : Invalid vertex name(s)
In addition: Warning message:
In as.igraph.vs(graph, v) : NAs introduced by coercion
```