function PrePrune(const b: array of TIndexRecord): double;
Pruning disables some attributes, to improve classificaiton accuracy.
Returns: double
Remarks:
Disabling some attributes attributes helps reducing noise in the response of the classes. The noise usually raises with the number of different attributes. All attributes are first disabled and then enabled one by one, to check, if this improves the classification accuracy. This process is called pre-prunning. The method calls the ClassifyTest method. If not implemented the results need to be returned via the OnClassifyTest event.