code: def add_cell_vector(self, vector):
          
          act = self.active_inactive[0]
          if len(act) == 3:
              raise ValueError("The unit cell already has three active cell vectors.")
          matrix = np.zeros((3, 3), float)
          active = np.zeros(3, bool)
          if len(act) == 0:
              # Add the new vector
              matrix[:, 0] = vector
              active[0] = True
              return UnitCell(matrix, active)
  
          a = self.matrix[:, act[0]]
          matrix[:, 0] = a
          active[0] = True
          if len(act) == 1:
              # Add the new vector
              matrix[:, 1] = vector
              active[1] = True
              return UnitCell(matrix, active)
  
          b = self.matrix[:, act[1]]
          matrix[:, 1] = b
          active[1] = True
          if len(act) == 2:
              # Add the new vector
              matrix[:, 2] = vector
              active[2] = True
              return UnitCell(matrix, active)
docs: Returns a new unit cell with an additional cell vector