lists all possible signed triangles

## Examples

```
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 2 4 1
#> [2,] 1 2 3 1
#> [3,] 1 3 4 3
#> [4,] 2 3 4 1
```

lists all possible signed triangles

```
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 2 4 1
#> [2,] 1 2 3 1
#> [3,] 1 3 4 3
#> [4,] 2 3 4 1
```