Turns a partial ranking into a directed graph. An edge (u,v) is
present if `P[u,v]=1`

, meaning that u is dominated by v.

`dominance_graph(P)`

- P
A partial ranking as matrix object calculated with neighborhood_inclusion or positional_dominance.

Directed graph as an igraph object.

```
library(igraph)
g <- threshold_graph(20, 0.1)
P <- neighborhood_inclusion(g)
d <- dominance_graph(P)
if (FALSE) {
plot(d)
}
# to reduce overplotting use transitive reduction
P <- transitive_reduction(P)
d <- dominance_graph(P)
if (FALSE) {
plot(d)
}
```