Resets all counters, truth and classification masks.

def reset_counters(self):
        """
        Resets all counters, truth and classification masks.
        """
        self.truth_masks = None
        self.classification_masks = None
        self.pixel_truth_counts = None
        self.pixel_classification_counts = None
        self.pixel_truth_sum = None
        self.pixel_classification_sum = None
        self.val_df = None
        self.n_validation_sets = 0