Created the MoleculeList.is_empty() method.
Updated the MoleculeList.add_item() method.
Added a docstring to the MoleculeList.__repr__() method.
Created the MoleculeContainer.is_empty() method.
Fixed a comment in the ResidueContainer.__repr__() method.
Updated the MoleculeContainer.__repr__() method.
The ResidueList add_item() method now calls the is_empty() method.
The SpinList add_item() method now uses the is_empty() method.
Created the ResidueList.is_empty() method.
Created the SpinList.is_empty() method.