TBD:
- add 'missing' column to $rules in output 
- add predict.xgrove method by calling predict(object$surrogate_grove, newdata)

xgrove_0.1-13 2025-09-15  - Bugfix for case remove.target = F  (ln 85ff).  
xgrove_0.1-12 2025-09-14  - preliminary test whether model has a terms component or attribute (ln 85ff).  
xgrove_0.1-11 2025-08-22  - Minor correction due to R winbuilder test.  
xgrove_0.1-10 2025-08-22  - remove.target argument added (using terms(model)) according to suggesiion from A.Zeileis.  
xgrove_0.1-9  2025-05-11  - Example added for classification where a predict function has to be defined.  
xgrove_0.1-8  2023-11-04  - Example added for classification where a predict function has to be defined.  
xgrove_0.1-7  2023-09-12  - Changes according to CRAN check: reference added to description, vlaue added to .Rd files plot.sgtree, plot.xgrove, upsilon  
xgrove_0.1-6  2023-09-11  - Title field changed to title case according to note from r-winbuilder.
xgrove_0.1-5  2023-09-08  - ln.1 added according to 'vo visible binding' note referring to ln.123 / group_by
xgrove_0.1-4  2023-09-07  - Arguments shrink = 1, b.frac = 1 added to xgrove() in order to change defaults of gbm() call.
xgrove_0.1-3  2023-05-15  - Function sgtree() added to compute surrogate trees.