Variable lower and upper bounds of a model

## Value

a list with two components 'lower' and 'upper' each having a numeric vector of bounds. One for each variable.

## Examples

```
library(magrittr)
model <- MIPModel() %>%
add_variable(x, type = "binary") %>%
add_variable(y, type = "continuous", lb = 2) %>%
add_variable(z, type = "integer", ub = 3)
variable_bounds(model)
#> $lower
#> [1] 0 2 -Inf
#>
#> $upper
#> [1] 1 Inf 3
#>
```