Not correct _is_greater_better func in tuner_interface.py
Created by: gorokhovnik
In tuner_interface.py function _is_greater_better does not guarantee correct output.
Example 1: The area under the precision-recall curve for unbalanced data might be less than 0,5 and the function will return False, while metrics needs to be maximized
Example 2: Custom metrics that needs to be minimized is not obliged to converge to zero