function PostPrune(const b: array of TIndexRecord): double;
Pruning disables some attributes, to improve classification 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 enabled and the disabled one by one, to check, if this improves the classification accuracy. This process is called post-prunning.