Keep simulations for a factorial experiment

## Arguments

- l
A list from read_apsim with Factorial.Permutation as root.

- s
a list with factor as name and levels as value to keep. The factor is kept if it is not specified.

## Examples

```
wheat <- read_apsimx(system.file("extdata/wheat.apsimx", package = "rapsimng"))
permutation <- search_path(wheat, path = "[Factors].Permutation")
permutation_new <- keep_simulations(permutation$node, list(V = "2"))
get_simulations(permutation_new)
#> $V
#> [1] "2"
#>
#> $Cv
#> [1] "Axe" "Bolac" "Calingiri"
#>
#> $N
#> [1] "0" "200"
#>
permutation_new <- keep_simulations(permutation$node, list(Cv = c("Axe", "Bolac")))
get_simulations(permutation_new)
#> $V
#> [1] "1" "2"
#>
#> $Cv
#> [1] "Axe" "Bolac"
#>
#> $N
#> [1] "0" "200"
#>
permutation_new <- keep_simulations(permutation$node,
list(V = "1", Cv = c("Axe", "Bolac")))
get_simulations(permutation_new)
#> $V
#> [1] "1"
#>
#> $Cv
#> [1] "Axe" "Bolac"
#>
#> $N
#> [1] "0" "200"
#>
```