Get the mean distance of a graph, which is the average path length in the graph. This operates through calculation of the shortest paths between all pairs of nodes.

## Examples

```
# Create a cycle graph
graph <-
create_graph() %>%
add_cycle(n = 5)
# Determine the mean distance
graph %>%
get_mean_distance()
#> [1] 2.5
# Create a full graph and then
# get the mean distance value
create_graph() %>%
add_full_graph(n = 10) %>%
get_mean_distance()
#> [1] 1
```