docs: def extend(self, other):
          """
          Method to extend the dataset vertically (add samples from  anotehr dataset).
  
          Parameters
          ----------
          other : MLDataset
              second dataset to be combined with the current
              (different samples, but same dimensionality)
  
          Raises
          ------
          TypeError
              if input is not an MLDataset.
          """
  
          if not isinstance(other, MLDataset):
              raise TypeError('Incorrect type of dataset provided!')
          # assert self.__dtype==other.dtype, TypeError('Incorrect data type of features!')
          for sample in other.keys:
              self.add_sample(sample, other.data[sample], other.labels[sample],
                              other.classes[sample])