Get the reciprocity of a directed graph. The reciprocity of a graph is the
fraction of reciprocal edges (e.g., `1`

-> `2`

and `2`

-> `1`

) over all edges
available in the graph. Note that for an undirected graph, all edges are
reciprocal. This function does not consider loop edges (e.g., `1`

-> `1`

).

## Examples

```
# Define a graph where 2 edge definitions
# have pairs of reciprocal edges
graph <-
create_graph() %>%
add_cycle(n = 3) %>%
add_node(
from = 1,
to = 1) %>%
add_node(
from = 1,
to = 1)
# Get the graph reciprocity, which will
# be calculated as the ratio 4/7 (where
# 4 is the number reciprocating edges
# and 7 is the total number of edges
# in the graph)
graph %>%
get_reciprocity()
#> [1] 0.5714286
# For an undirected graph, all edges
# are reciprocal, so the ratio will
# always be 1
graph %>%
set_graph_undirected() %>%
get_reciprocity()
#> [1] 1
# For a graph with no edges, the graph
# reciprocity cannot be determined (and
# the same NA result is obtained from an
# empty graph)
create_graph() %>%
add_n_nodes(n = 5) %>%
get_reciprocity()
#> [1] NA
```