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 ae41954 UN-- 3 3 --
#> + attr: name (v/c), type (e/c)
#> + edges from ae41954 (vertex names):
#> [1] a--b a--c b--c
```