`EProblemType` has more than `PT_REGRESSION` and `PT_MULTICLASS` as possible values. Make this better a `swith`, deal with the specific problem types for which CART is defined (`PT_MULTICLASS` and `PT_REGRESSION` if I understand everything correctly), and make a `default` branch with an error to handle properly every possibility (even if the setter does not really allow to put other problem types, I don't know if that is the case though).