From a graph object of class `dgr_graph`

, get a count of all loop edges in
the graph.

## Examples

```
# Create an undirected, full graph
# with 3 nodes and all possible
# edges, including loop edges
graph <-
create_graph(
directed = FALSE) %>%
add_full_graph(
n = 3,
keep_loops = TRUE)
# Get a count of all loop edges
# in the graph
graph %>% count_loop_edges()
#> [1] 3
```