Refactoring `Bond` class...
Refactoring `Bond` class...

...including:

* fixing `__eq__` and `__lt__` methods
* adding property setter methods for `atoms` and `vector`
  attributes.
* adding a `fmtstr` attribute and corresponding property
  getter/setter methods
* adding a `todict` method

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