Determines whether a graph is set to be directed or not and returns a logical value to that effect.

## Examples

```
# Create an empty graph; by default,
# new graphs made by `create_graph()`
# are directed
graph <- create_graph()
# Determine whether the graph
# is directed
graph %>% is_graph_directed()
#> [1] TRUE
# Use the `set_graph_undirected()`
# function and check again whether
# the graph is directed
graph %>%
set_graph_undirected() %>%
is_graph_directed()
#> [1] FALSE
```