User defined cuts for tree-based methods

[wbuchanan]

Feature

It isn't currently clear how step_collapse_cart and other methods for numeric predictors determine how many groups to select. It could be useful to add a parameter cut_depth that would allow users to specify how far down the decision tree to make cuts to collapse/group values. Alternatively, a groups parameter that allows users to specify the number of groups they want to retain, which would return <= groups based on cutting the tree could also be a useful feature for users.