lists all possible signed triangles

`signed_triangles(g)`

- g
igraph object with signed edge attribute

matrix of vertex ids and the number of positive ties per triangle

```
library(igraph)
g <- graph.full(4)
E(g)$sign <- c(-1,1,1,-1,-1,1)
signed_triangles(g)
#> V1 V2 V3 P
#> [1,] 1 5 3 1
#> [2,] 1 4 2 1
#> [3,] 2 6 3 3
#> [4,] 4 6 5 1
```