Determines whether a graph is a connected graph.

## Examples

```
# Create a random graph using the
# `add_gnm_graph()` function; this
# graph is not connected
create_graph() %>%
add_gnm_graph(
n = 15,
m = 10,
set_seed = 23) %>%
is_graph_connected()
#> [1] FALSE
# Create another random graph;
# this graph is connected
create_graph() %>%
add_gnm_graph(
n = 10,
m = 15,
set_seed = 23) %>%
is_graph_connected()
#> [1] TRUE
```