From a graph object of class `dgr_graph`

, get a count of nodes in the graph
that are not connected to any other node.

## Examples

```
# Create a graph with a
# path of nodes and 3
# unconnected nodes
graph <-
create_graph() %>%
add_path(n = 3) %>%
add_n_nodes(n = 3)
# Get a count of all nodes
# in the graph
graph %>% count_nodes()
#> [1] 6
# Get a count of all
# unconnected nodes in the
# graph
graph %>%
count_unconnected_nodes()
#> [1] 3
```