This function determines the minimal depth of variables from a forest that
is created by [getTreeranger()] and [addLayer()].
Arguments
- variables
vector of variable names
- trees
list of trees that was generated by [getTreeranger] and [addLayer()].
Value
List with the following components:
* `depth`: mean minimal depth for each variable
* `selected`: variables has been selected (1) or not (0),
* `threshold`: the threshold that is used for the selection