Get the girth of a graph, which is the length of the shortest circle in the graph. Loop edges and multiple edges are not considered. If the graph contains no cycles then zero is returned.

## Examples

```
# Create a cycle graph
graph <-
create_graph() %>%
add_cycle(n = 5)
# Determine the graph's girth
graph %>% get_girth()
#> [1] 5
# Create a full graph and then
# get the girth for that
create_graph() %>%
add_full_graph(n = 10) %>%
get_girth()
#> [1] 3
```