Gets the column duals of a solution
Examples
if (FALSE) {
result <- MIPModel() %>%
  add_variable(x[i], i = 1:5) %>%
  add_variable(y[i, j], i = 1:5, j = 1:5) %>%
  add_constraint(x[i] >= 1, i = 1:5) %>%
  set_bounds(x[i], lb = 3, i = 1:3) %>%
  set_objective(sum_over(i * x[i], i = 1:5)) %>%
  solve_model(with_ROI("glpk"))
get_column_duals(result)
}