subgroup_perm_test: permutation test for depth 1 trees
In molson2/subgroupTree: Subgroup Detection Trees with Inference

Perform a permutation test for a null-effect for the largest ATE discovered from a search using a depth-1 subgroup tree.

1 2	subgroup_perm_test(response, treated, X, direction = c("max", "min"), n_perm = 1000, ate = 0, minbucket = 100)

`response`	numeric outcome of interest
`treated`	boolean vector of treatment assignments
`X`	data.frame of predictors; must be either numeric or factor types
`direction`	"max" to search for largest treatment effect, "min" to search for smallest treatment effect
`n_perm`	number of permutations to calculate
`ate`	null average treatment effect to test against
`minbucket`	minumum number of observations required to consider a split