docs: def extend(self, assembly):
          """Extends the `Assembly` with the contents of another `Assembly`.
  
          Raises
          ------
          TypeError
              Raised if other is any type other than `Assembly`.
          """
          if isinstance(assembly, Assembly):
              self._molecules.extend(assembly)
          else:
              raise TypeError(
                  'Only Assembly objects may be merged with an Assembly.')
          return