code: def accuracy(self):
          
          sub_observed = np.array([self.observed.metadata[i] for i in self.observed.arr])
          return float((self.model_predictions() == sub_observed).sum()) / self.data_size
docs: Calculates the accuracy of the tree by comparing
          the model predictions to the dataset
          (TP + TN) / (TP + TN + FP + FN) == (T / (T + F))