Determines whether a graph is set as undirected 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, so, use
# `directed = FALSE` to create
# an undirected graph
graph <-
create_graph(
directed = FALSE)
# Determine whether the
# graph is undirected
graph %>% is_graph_undirected()
#> [1] TRUE
# Use the `set_graph_directed()`
# function and check again
# as to whether the graph is
# undirected
graph %>%
set_graph_directed() %>%
is_graph_undirected()
#> [1] FALSE
```