Adding modified implementation of Python 3's `UserList` class.
Adding modified implementation of Python 3's `UserList` class.

Replacing my custom base classes for `AtomList` and `BondList`
with a modification of the `UserList` class from Python 3.

The `AtomList` and `BondList` class now subclass the `UserList`.

scikit-nano/scikit-nano,androomerrill/scikit-nano,androomerrill/scikit-nano,scikit-nano/scikit-nano