Convert statistics method to ave, min, and max.

def convertstatsmethod(method_str):
        """Convert statistics method to ave, min, and max."""
        if StringClass.string_match(method_str, 'Average'):
            return 'ave'
        elif StringClass.string_match(method_str, 'Maximum'):
            return 'max'
        elif StringClass.string_match(method_str, 'Minimum'):
            return 'min'
        elif method_str.lower() in ['ave', 'max', 'min']:
            return method_str.lower()
        else:
            return 'ave'