Counts the number of all possible signed triangles (+++),(++-), (+--) and (---)

`count_signed_triangles(g)`

- g
igraph object with signed edge attribute

counts for all 4 signed triangle types

```
library(igraph)
g <- graph.full(4)
E(g)$sign <- c(-1,1,1,-1,-1,1)
count_signed_triangles(g)
#> +++ ++- +-- ---
#> 1 0 3 0
```