convert signed two-mode network to unsigned

## Arguments

- g
igraph object. Two-mode network, must have a "sign" edge attribute.

- primary
logical. Which mode to transform

## Examples

```
library(igraph)
# create a simple signed two mode network
el <- matrix(c(1, "a", 1, "b", 1, "c", 2, "a", 2, "b"), ncol = 2, byrow = TRUE)
g <- graph_from_edgelist(el, directed = FALSE)
E(g)$sign <- c(1, 1, -1, 1, -1)
V(g)$type <- c(FALSE, TRUE, TRUE, TRUE, FALSE)
# convert to unsigned two-mode network and project
l <- as_unsigned_2mode(g, primary = TRUE)
p <- bipartite_projection(l, which = "true")
# turn the unsigned projection back to a signed network
as_signed_proj(p)
#> IGRAPH 5a99865 UN-- 3 3 --
#> + attr: name (v/c), type (e/c)
#> + edges from 5a99865 (vertex names):
#> [1] a--b a--c b--c
```