convert unsigned projection to signed

## Examples

```
library(igraph)
#>
#> Attaching package: ‘igraph’
#> The following objects are masked from ‘package:stats’:
#>
#> decompose, spectrum
#> The following object is masked from ‘package:base’:
#>
#> union
# 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 12b2e72 UN-- 3 3 --
#> + attr: name (v/c), type (e/c)
#> + edges from 12b2e72 (vertex names):
#> [1] a--b a--c b--c
```